Bloom Continuous Casting Research Articles

A New Approach to Calculate the Velocity of Interdendritic Fluid Flow during Solidification Using Etched Surface Height of Actual Metal Ingot

Macrosegregation remains one of main defects affecting metal materials properties, which is mainly caused by interdendritic fluid flow during solidifying. However, as for controlling actual specific segregation, it is still difficult to effectively measure or simulate this kind flow instead of pure fluid flow, especially in complex casting processes of high-grade materials. Herein, a new method for obtaining velocity magnitude and direction of interdendritic fluid flow during metal solidifying is proposed from boundary layer and standard deviation obtained by measuring etched surface heights of the actual ingot and using statistical principles. Taking continuous casting bloom of GCr15 bearing steel as an example, it is indicated that the calculated velocity magnitudes under different sides and superheats can be explained by process features and, hence, solidification mechanism. The velocity magnitude and fluctuation are higher on the inner curve side and under low superheat. Meanwhile, it is found that the fluctuation extent of secondary arm spacing is more relevant with interdendritic fluid flow, although its magnitude is mainly determined by the cooling rate. Moreover, on the basis of the calculated velocity directions and magnitudes, there is a positive correlation between segregation area ratio and the effective ratio between interdendritic flow velocity and growth velocity especially in the equiaxed grain zone, which corresponds with classic macrosegregation formation theory. The above findings and comparison with other results demonstrate the validity of the new approach, which can obtain the magnitude and the direction of interdendritic fluid velocity for two or three-dimensional multiscale velocity distribution by tailoring measuring length and numbers.

Mathematical simulation of two-phase flow and slag entrainment during steel bloom continuous casting

A three-dimensional computational model, coupled with the Large Eddy Simulation (LES) model and Volume of Fluid (VOF) multiphase model, was established to investigate the two-phase flow and slag entrainment during steel bloom continuous casting. The influence of the in-mold electromagnetic stirring (M-EMS), casting speed, and submerged entry nozzle (SEN) immersion depth on the flow pattern and slag entrainment were evaluated. The number, size, velocity, and spatial distribution of slag droplets were counted and the location, velocity and probability of the slag entrainment on the meniscus were exported in real time through a user defined function. The result shows that the slag entrainment mainly occurred around the 1/4 width of the mold. The equivalent diameter of most slag droplets was 2–6 mm under the current casting condition. A formula for the equivalent diameter distribution of slag droplets was proposed according to the statistical result.

Effect of Mold Electromagnetic Stirring and Final Electromagnetic Stirring on the Solidification Structure and Macrosegregation in Bloom Continuous Casting

The solidification structure and macrosegregation are investigated in different current intensities of mold electromagnetic stirring (M‐EMS) and final electromagnetic stirring (F‐EMS) by a series of industrial trials during the 20CrMnTi bloom continuous casting (CC) process. With the current intensity of M‐EMS increasing from 100 to 390 A, the deflection angle of columnar dendrites increases from 25.0 to 32.49 deg. With the current intensity of M‐EMS increasing from 0 to 390 A, the equiaxed crystal ratio increases from 20.4% to 25.7% and center porosity varies little. With the increasing current intensity of M‐EMS, the negative segregation in the subsurface and positive segregation in the columnar‐to‐equiaxed transition (CET) zone deteriorate gradually. The M‐EMS has little effect on the center‐positive segregation, and the center‐positive segregation is maintained at 1.37–1.40. With the current intensity of F‐EMS increasing from 0 to 500 A, the equiaxed crystal ratio is within 25.3–25.8%, and the large center porosities disappear instead of numerous fine spots in the equiaxed zone. The degree of center‐positive segregation decreases from 1.48 to 1.30. For the CC process, to obtain a better internal quality of 20CrMnTi bloom, the optimal current intensities of M‐EMS and F‐EMS are 0 and 500 A, respectively.

Control of Shrinkage Porosity and Spot Segregation in Ø195 mm Continuously Cast Round Bloom of Oil Pipe Steel by Soft Reduction

Based on the Ø195 mm round bloom continuous casting of oil pipe steel, a two dimensional thermal-mechanical coupled model has been developed to investigate the deformation behavior of round bloom during soft reduction (SR) in the reduction force mode. Good agreement was achieved in surface temperature, shell thickness and contact zone width from modeling and measurement. Under the same reduction force, the reduction amount of round bloom at the front unit is much larger than back unit. Moreover, due to its higher temperature and lower center solid fraction, the deformation penetration before solidification is much stronger than that after solidification. Considering the limitation of the round bloom ovality, the maximum allowable force in reduction unit is calculated. According to the simulation results, a multi-unit soft reduction plan was proposed and carried out on the Ø195 mm round bloom. After the reduction process of No.1 to No.3 withdrawal units, the shrinkage porosity in the center of the round bloom was almost vanished, while the number and size of spot segregation were significantly reduced. Moreover, the oil pipe produced by the round bloom with SR got a better resistance to sulfide stress corrosion (SSC). It indicates that SR is an effective technology for the round bloom to control the shrinkage porosity and spot segregation in the continuous casting.

Numerical Modeling of the Macrosegregation Improvement in Continuous Casting Blooms by Using F-EMS

A three-dimensional mathematic model based on segmentation that joins the electromagnetic field, melt flow, heat transfer, solidification and solute transport in the whole casting domain has been established to study the solute transport at the final stage of solidification in a bloom continuous casting process. The effects of the final electromagnetic stirring (F-EMS) positions on the macrosegregation degree of the as-cast bloom are compared and analyzed. The results show that the maximum carbon segregation degree, the ratio of the local carbon concentration to the initial carbon concentration, is reduced from 1.292 to 1.254 with the application of F-EMS. In addition, the position of F-EMS is also an important factor. The maximum carbon segregation degrees are 1.254, 1.237 and 1.269 when the installation location of F-EMS is 14 m, 16 m and 18 m away from the meniscus, respectively. The optimal center solid fraction is about 0.1 at the F-EMS center for this study.

Feasibility of using Continuously Cast Round Bloom as a Substitute to Cast Ingot in the Manufacture of Heavy Forgings

Continuously cast large round blooms have gained widespread attention due to the improved quality, enhanced economy, higher yields, and lower operating costs obtained through continuous operation. Because the continuous casting (CC) process combines the cleanliness and radial solidification symmetry of the ingot casting process with the higher axial symmetry obtainable through bloom casting, it can deliver a more homogeneous product than conventional ingot casting. Herein, the shop floor manufacturing of a main shaft is discussed to confirm the feasibility of using continuously cast round bloom in the manufacture of heavy forgings. For comparison, the characteristics of a 1000 mm‐diameter continuously cast round bloom are assessed against those of a conventionally cast 20.6 ton ingot. To eliminate cavities at the centerline of the cylindrical bloom, additional studies on void closure efficiency at the center of the billet under various forging die geometries (e.g., flat, V, and round dies) are conducted, and the results reveal that the V‐die geometry is most effective in consolidating centerline voids. A comparison of the respective operational characteristics reveals that the round bloom CC method could be feasibly applied in the manufacture of main shafts.

Prediction on the spatial distribution of the composition of inclusions in a heavy rail steel continuous casting bloom

Industrial trials were performed to investigate the evolution of inclusions in the molten steel of the continuous casing tundish and the bloom of a heavy rail steel, and the spatial distribution of inclusion composition along the bloom thickness was studied. An integrated model was established to predict the spatial distribution of the inclusion composition on the entire cross-section of a heavy rail steel continuous casting, which coupled the heat transfer and solidification of steel during continuous casting, the thermodynamic transformation of inclusions with temperature and the kinetic diffusion of dissolved elements in the steel. Both industrial trials and model predictions achieved similar conclusions.Inclusions within the 30mm subsurface layer had a similar composition and size to those of the molten steel and were more than other locations of the bloom and at 1/4 thickness of the bloom inclusions were larger and contained the biggest concentration of CaS. Along the thickness of the bloom, an opposite fluctuation of CaO to that of CaS in inclusions existed so that the main reaction between the inclusion and the steel matrix was (CaO)+[S]=(CaS)+[O]. When the cooling rate was larger than 150K/s, the newly precipitated CaO–Al2O3–MgO–SiO2–(<5 pct CaS) inclusions averaged <1.5μm in diameter. If the cooling rate was smaller than 150K/s, inclusions were transformed to larger CaO–Al2O3–MgO–SiO2–(>10 pct CaS) ones. In order to achieve an optimized composition of inclusions, the total oxygen and aluminum in the steel should be <10ppm and <40ppm.

Growth of solidified shell in bloom continuous casting mold of hypo-peritectic steel based on a FeS tracer method

Solidification behavior in the mold region plays an important role in production efficiency and steel quality. To investigate shell growth within a mold, the sulfur prints of the entire shell thickness profile from the meniscus to 100 mm below the mold were obtained by adding FeS tracer into molten steel during bloom continuous casting of hypo-peritectic steel. The law of shell thickness evolution along mold height and circumference was analyzed. The results show that there are three weak regions of solidification, which are in the mold upper part, in the mold lower part, and just below mold exit, possibly resulting from periodic fluctuation of air gap between the shell and the mold, the impingement of melt jets on the solidification front, and the decreasing cooling intensity, respectively. Initial solidification point along casting direction appears at approximately 35 mm below the meniscus. Overall, the solidified shell thickness in the inner side of the mold is a little larger than that in the outer side, and the former and the latter reach 25.5 and 24.3 mm at the mold exit, respectively. The non-uniform shell growth in the inner side of the bloom is provided, while shell thicknesses in the narrow face and the outer side follow relatively regular growth. Out of the mold, the thinnest shells on the transverse section exist in the regions of 60–90 mm and 40–70 mm from the corners of the inner and outer sides, respectively, i.e., the off-corners.

Modeling the Effect of Combined Electromagnetic Stirring Modes on Macrosegregation in Continuous Casting Blooms

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.

A review of chamfer technology in continuous casting process

Chamfer technology, with funnel-shaped curved surface on the narrow side of the mold, is a novel technology that can effectively improve the quality of continuously cast products. This study reviews the available literature on the theoretical and applied research in chamfer technology to provide an in depth analysis of the employed approaches and the obtained results. According to theoretical research results, combined with the working conditions of slab caster, patented technologies and related equipment were developed. The research has broken the technology bottlenecks of the industrial application, while ensuring a long-life operation of the chamfer mold. In recent years, chamfer technology, which is used in slab casting processes, can prevent transverse corner cracks to form at the slab surface of micro-alloyed steel completely. Chamfer technology, which is used in thin slab casting processes, can reduce the occurrence of longitudinal surface crack of the slabs and straight edge seam defects of the rolled strip. In addition, chamfer technology, which is used in a continuous casting bloom, can reduce the risk of internal cracks and avoid the off-corner cracks occurring in as-cast bloom.

Research on several constitutive models to predict the flow behaviour of GCr15 continuous casting bloom with heavy reduction

In this work, the modified Johnson-Cook (J-C) model was developed. The predictive abilities of the JC model, new modified JC model, modified Zerilli-Armstrong (ZA) model, and strain-compensated Arrhenius model of the flow behaviour of GCr15 continuous casting bloom with heavy reduction (HR) were compared. The deformation mechanism was explored. Experimental stress-strain data from single-pass thermo-simulation compression experiments over a wide range of temperatures (1023–1573 K), strains (0.05–0.7) and strain rates (0.001–0.1 s−1) were applied to deduce the material parameters of these constitutive models. The applicability of the models was analysed by calculating the correlation coefficient; the average relative error (AARE) of the model; and the number of model parameters, which represents the computation time of each model. The experimental results showed that the new modified JC model was more suitable for describing the flow behaviour of GCr15 continuous casting bloom with HR because of its convenient calculation and better accuracy.

Effect of the Gap Between Copper Mold and Solidified Shell on the Fluid Flow in the Continuous Casting Strand with Mold Electromagnetic Stirring

Considering the gap between the copper mold and the solidified shell (mold‐bloom gap), the electromagnetic field, fluid flow, solidification, removal of inclusions, and slag entrainment in the bloom continuous casting process with mold electromagnetic stirring (M‐EMS) are numerically investigated. As the mold‐bloom gap is considered, the electromagnetic force increases with the distance from the meniscus and reaches a maximum value near the stirrer center. With the distance further increasing, the electromagnetic force decreases continuously. With the mold‐bloom gap ignored, there are two peaks of the electromagnetic force along the distance below the meniscus. One is 40 mm above the stirrer center and the other is near the mold exit. When the mold‐bloom gap is considered, the velocity magnitude is higher and the temperature field on meniscus is more uniform. The net slag entrainment rate is 0.0144 kg s−1 and is higher than that ignoring the mold‐bloom gap. Inclusions are more removed with considering the mold‐bloom gap.

Closure of Internal Porosity in Continuous Casting Bloom During Heavy Reduction Process

To investigate the closure behavior of internal porosity (also referred to as internal void) in continuous casting bloom during heavy reduction (HR) and thus provide theoretical guidance for minimizing this kind of internal defect more effectively with HR, a three-dimensional (3D) mechanical model was developed based on the predicted temperature field by a 2D heat transfer model. With this 3D mechanical model, closure behaviors of internal porosity in continuous casting bloom during HR at and after the strand solidification end under different process conditions were numerically studied. It was found that the void axis length decreased significantly along the bloom thickness direction and increased slightly along the casting and bloom width directions after HR, and the influence of the initial void size on the void closure was not obvious. With a decrease of temperature difference between the bloom surface and center, HR efficiency for minimizing internal void decreased, while the required reduction force significantly increased. Compared with blooms with a uniform temperature distribution of 1100 °C, the void closure index after HR implemented at the strand solidification end was increased by ~ 25 pct. Compared with a conventional flat roll, the application of a convex roll during HR could contribute to minimizing the internal porosity more effectively and significantly enhance the reduction capacity of the withdrawal and straightening units. The void closure index of ηs and ηv (where ηs and ηv were defined based on the variation of the void aspect ratio and the void volume, respectively) was closely related to the equivalent strain (eeq) and the hydrostatic integration parameter (Q), respectively, and two mathematical equations were derived to quantitatively describe the relationship of ηs − eeq and ηv − Q.

Analysis on Electromagnetic Field of Continuous Casting Mold Including a New Integral Method for Calculating Electromagnetic Torque

Based on the Maxwell’s equations, a finite element model is established to study the characteristics of electromagnetic field in the mold of billet and bloom continuous casting with electromagnetic stirring (M-EMS). A novel integral method for calculating electromagnetic torque is proposed to evaluate the stirring intensity of stirrer. In order to verify the accuracy of the model, a well-designed electromagnetic torque detecting device is fabricated. The predicted value of electromagnetic torque and magnetic flux density are consistent with the measured data. The optimum frequency is determined by the maximal electromagnetic torque of the strand. The effect of stirring current intensity and different stirrer positions along the length of mold on the electromagnetic field has been studied numerically. The results show that the optimum frequency is smaller when the copper tube of the mold is thicker and the section size is bigger. Besides, the electromagnetic torque of the strand is a quadratic function of the running current intensity. Moreover, the installation position of stirrer strongly affects the prediction of electromagnetic field distribution, further influencing the optimum frequency and the electromagnetic torque of strand.

Numerical simulations of liquid steel alloying in the three strand continuous casting bloom tundish

This paper aims to obtain fundamental information on chemical homogenisation process of liquid steel with alloy additions in the bloom tundish. For alloy feeding to liquid steel pulse step alloying method was applied. Author checked the effect of hydrodynamic conditions occurs in the internal working space of tundish on the process of alloy mixing with liquid steel. Within the work basic and proposed tundish equipments were considered. Numerical modelling technique was employed to demonstrate the process of alloy addition mixing during continuous casting process. Ansys-Fluent computer program was used for numerical simulation. For particular continuous casting strands time mixing was calculated.

Influence of differential roll rotation speed on evolution of internal porosity in continuous casting bloom during heavy reduction

Heavy reduction (HR), during which a large reduction deformation is implemented around the strand solidification end during continuous casting, is a novel technology that could effectively improve the internal quality of large section continuous casting steel. In the present work, a 3 dimensional (3D) thermal-mechanical coupled model with two pairs of rolls was developed. By setting different rotation speed of the front and the reduction rolls, the effect of differential roll rotation speed on the evolution of internal porosity in continuous casting bloom during HR was investigated. Initial temperature distribution of the 3D thermal-mechanical coupled model was determined by the predicted heat transfer results of a 2 dimensional (2D) heat transfer model. An Arrhenius-type constitutive equation, derived based on the measured true stress-strain curves under different temperatures and strain rates, was adopted in the 3D thermal-mechanical coupled model to describe the metal flow behavior of the casting strand during HR. Accuracy of the 3D thermal-mechanical coupled model was verified by the feedback pressure of the hydraulic cylinders during plant trial of HR. The simulation results indicate that a relatively larger rotation speed of the front rolls (Vf) than that of the reduction rolls (Vr) could effectively suppress the porosity elongation along the casting direction and the bloom width direction, and benefit the decrease of porosity dimension along the bloom thickness direction. As a result, the effect of HR on improving the bloom internal porosity could be effectively promoted. With HR amount of the reduction roll (Rr) decreased or the small reduction amount of the front rolls (rf) increased, the improvement effect of differential roll rotation speed on HR becomes more significant. With roll rotation speed ratio of Rv (Rv = Vf / Vr) increased from 1.0 to 2.0, efficiency of differential roll rotation speed on promoting the process effect of HR continuously decreased.

Improvement of centre segregation in continuous casting bloom and the resulting carbide homogeneity in bearing steel GCr15

ABSTRACTIn the present work, solidification structure and centre segregation in continuously cast bearing steel GCr15 blooms with 220mm×260 mm were obtained by different casting conditions (M-EMS, M + F-EMS, M + F-EMS + MSR), which were systematically optimized by numerical simulation and experiments. The relationship between centre segregation in continuously cast GCr15 bloom, heredity in hot-rolled steel and carbide inhomogeneity of hot-rolled steel with was investigated comprehensively. Results showed that compared with M-EMS, centre carbon segregation ratio in the bloom decreased from 1.1∼1.4 to 1.04∼1.22 by M + F-EMS, and could be reduced to 0.96∼1.08 by MSR in combination with M + F-EMS. Moreover, centre segregation in hot-rolled steel decreased with the decrease of centre segregation in bloom. Grade of network and banded carbide in hot-rolled steel decreased accordingly, especially for centre carbon segregation ratio with 0.96∼1.08, grade of network and banded carbide could be significantly reduced to 1. It was important to control centre segregation in as-cast steel to improve the resulting carbide homogeneity of as-rolled steel. According to the experimental results, higher casting speed in bloom with M + F-EMS and MSR integrated process was preferred to effectively reduce centre segregation ratio and further decreased the occurrence of network and banded carbide in as-rolled bearing steel GC15.

Application of Numerical Model of Continuous Cast Bloom Crystallization to Improve the Efficiency of Mechanical Soft Reduction Technology

The paper presents the results of the analysis of thermal state of continuous cast bloom on the stage of incomplete crystallization in the area of possible places of application of technology of Mechanical Soft Reduction (MSR), received during simulation on mathematical model, which was implemented in the software package using the finite element method. For the conditions of continuous cast bloom with a cross section of 360-400 mm, we studied the characteristics of temperature change over the cross section of the bloom, the growth dynamics of the solid shell, the number of the solidified component, and the end point position of solidification depending on the technological parameters for continuous casting of A516, 5140 1070 and AISI steel brands. The obtained data on the thickness of the crystallized shell of continuous cast bloom and the amount of solid phase depending on the casting speed and steel grade allowed us to conclude about the possible place of installation of the MSR unit. The results of mathematical simulation can be transferred in a digital form to the strength model of deformation of continuous cast bloom at the stage of incomplete crystallization.

Experimental Study of the Mold Flow Induced by a Swirling Flow Nozzle and Electromagnetic Stirring for Continuous Casting of Round Blooms

This study focuses on an experimental investigation of the fluid flow in round bloom continuous casting using a 1:3 model of the industrial casting process. A swirling flow nozzle, represented by the specific design of the RHI Magnesita GYRONOZZLE, is used to produce a swirling motion in the cylindrical mold. The test section is integrated into the Mini-LIMMCAST facility at HZDR, which is operated at room temperature using the ternary alloy GaInSn. Systematic measurements of horizontal and vertical velocity profiles are performed by means of the Ultrasound Doppler Velocimetry. The second part of the study focuses on the interaction between the flow driven by the GYRONOZZLE and concurrent electromagnetic stirring in the mold (M-EMS) by applying rotating magnetic fields (RMFs) at different magnetic flux densities. The effect of the GYRONOZZLE on the flow pattern inside the mold is examined with and without superimposed RMFs and compared to those of a standard single-port nozzle. The measurements reveal that the GYRONOZZLE induces a swirling flow in the whole mold. It is further shown that the influence of a simultaneously applied RMF is mainly restricted to the lower part of the mold since the transport of angular momentum to the top is suppressed by the jets pouring out from the GYRONOZZLE.

Application of a chamfer slab technology to reduce internal cracks of continuous casting bloom during soft reduction process

The stress concentrations over the brittle temperature range (BTR) in the bloom continuous casting are the main reason of internal cracks. In order to analyze the stress distribution in the BTR of the blooms during soft reduction stage, a three-dimensional thermo-mechanical finite-element model with different corner structures (i.e. chamfer angle and chamfer length) was established. The relationship between corner structures, maximum tensile stress, as well as shear stress is analyzed, and the influence of corner structure of bloom on the internal cracks is studied. The results show that the tensile stress and the shear stress decreased gradually by properly adjusting the chamfer angle and the chamfer length of the bloom. Compared with the use of the right-angle bloom casting, the application of chamfer bloom casting is able to reduce the stress concentration over the BTR, therefore reduces the internal cracks. In addition, as a side benefit, the chamfer bloom casting can save energy required in deforming the bloom during the soft reduction process.