Round Bloom Research Articles

Numeric simulations of a liquid metal model of a bloom caster under the effect of rotary electromagnetic stirring

At the voestalpine Stahl Donawitz GmbH the continuous casting of round steel blooms is commonly supported by electromagnetically induced stirring of the liquid steel flow. A number of beneficial effects are attributed to electromagnetic stirring in the mould region (M-EMS), e.g. the enhanced transition from columnar to equiaxed solidification, the homogenization of the liquid steel flow or the reduction of surface and subsurface defects. Although the positive effects of M-EMS can be seen on the blooms (e.g. in etchings), the link between electromagnetic stirring of the steel melt and the quality of the solidified bloom is not sufficiently understood. Theoretical considerations are often limited to general cases and their results are therefore not directly applicable to real continuous casting geometries. On the other hand, plant measurements can only be performed to a limited extent due to the harsh conditions and other restrictions (e.g. safety regulations). In this work an alternative approach is used to investigate the steel flow in a round bloom caster under the influence of M-EMS. In a 1:3 scale Perspex model of a round bloom strand, measurements of the flow under the influence of a rotating magnetic field can be conducted. These measurements provide a validation benchmark for the numeric simulations. A numeric model of the before mentioned 1:3 scale model is implemented, encompassing the strand, the submerged entry nozzle as well as the M-EMS device. In the modelling approach, the bidirectional coupling between liquid steel flow and the electromagnetic field/forces has to be considered because otherwise the resulting tangential velocities will be overestimated. With the validated modelling approach, simulations of real casting machines can then be conducted, stirring parameter influences can be shown and conclusions for the real casting process can be drawn.

Numerical Simulation of Electromagnetic Field in a Round Bloom Continuous Casting Mould with Eccentric Electromagnetic Stirring

Numerical Simulation of Electromagnetic Field in a Round Bloom Continuous Casting Mould with Eccentric Electromagnetic Stirring

Numerical simulation on the solidification structure of Ø600mm continuous casting round bloom

A FE (Finite Element)—CA (Cellular Automation) coupling model was developed for the simulation of solidification structure formation during the Ø600mm round bloom continuous casting process of Q345E steel. The simulation result of the temperature field was consistent with the nail-shooting experimental result, and the simulated solidification structure of the bloom was in great agreement with corrosion testing under the same casting condition. The simulation results showed that the centre equiaxed crystal ratio increased slightly with the increase of secondary cooling water rate and decreased with the increase of casting temperature and casting speed. When the secondary cooling water rate was over 0.08L/kg, it had less effect on the solidification structure. As the casting temperature increased by 1°C or the casting speed increased by 0.01m/min, the centre equiaxed crystal ratio would decrease by 0.4%∼1.2% and 3%∼0.8% respectively. According to the simulation results, the optimized continuous casting process of Ø600mm round bloom should be the secondary cooling water rate of 0.08L/kg, the casting temperature of 1532°C∼1539°C and the casting speed of 0.20m/min∼0.22m/min. It was found that the solidification structure of Ø600mm Q345E steel round bloom was much improved after the optimized continuous casting process was adopted in practical production.

Formation and control of macrosegregation for round bloom continuous casting

Based on the developed coupled model of electromagnetism, heat and solute transportation, the macrosegregation formation and effect of secondary cooling water ratio on macrosegregation degree in strand during round bloom continuous casting process have been investigated. The solute segregation degree fluctuates from a positive to a negative value with distance from strand surface in the initial solidified shell region within thickness of 20 mm. A negative segregation region in concave shape and an irregular positive segregation zone are presented in the fixed and loosened side of strand respectively due to the gravity and thermosolutal convection. As the secondary cooling water ratio decreases from 0.25 to 0.15 L kg− 1, the solidification ratio at final electromagnetic stirring (F-EMS) centre increases from 73.14 to 77.83%. For the steel grade of 50Mn casted by round bloom casting within diameter of 0.35 m, the optimal solidification ratio at F-EMS centre is 75.05%, where the radial centre crack and shrinkage cavity at strand cross-section are removed.

Numerical Analysis of Coupled Turbulent Flow and Macroscopic Solidification in a Round Bloom Continuous Casting Mold with Electromagnetic Stirring

In this article, a coupled three‐dimensional mathematic model is established to describe electromagnetic field, fluid flow, heat transfer and solidification in a round bloom continuous casting mold with electromagnetic stirring (M‐EMS). The interaction between the induced flow and the impinging jet from a straight‐through submerged entry nozzle (SEN) of the caster is numerically analyzed. The results show that the electromagnetic force appears to be circinate at the cross section of the round bloom. Moreover, the flow of the melt is characterized by a dominant swirling movement along the azimuthal direction in the horizontal plane with M‐EMS. However, the swirl flow velocity decreases remarkably when solidification is taken into account. With the increase of stirring intensity, the steady flow becomes unstable accompanying with the bias flow, and the temperature distribution is unsymmetrical in the mold. The significant swirl flow with M‐EMS prevents the superheated jet moving downward and weakens the invasion depth of the jet, and thus the location of the hot zone in the mold moves up evidently.

Soft Reduction at a Round Bloom Caster: Implementation and Results

As a consequence of steadily increasing demands on the control of macro segregation in high quality steel products, voestalpine Stahl Donawitz GmbH started investigations on the application of the soft reduction technique for the five strand round bloom caster CC3. Between the years 2006 and 2010, a broad variety of different plant trials with steel grades ranging from 0.8 to 1.05 %C were conducted. The experiments started with the given pinch roll configuration at the pilot strand number one and finally provided a wide understanding of the influence of soft reduction on the solidification structure and on the concentration level in the core region. As a part of this project, a time and cost efficient method for the quantification of the center segregation was established. In addition, numerous segregation values were collected along the entire process route, from bloom to wire. As a consequence of the outstanding results, all five strands were equipped with a soft reduction unit in 2010.

Erratum to: A Framework for Soft Sensing of Liquid Pool Length of Continuous Casting Round Blooms

JIAN YANG, Ph.D. Student, ZHI XIE and JING NING, Professors, are with the College of Information Science and Engineering, Northeastern University at Shenyang, Liaoning, P.R. China 110819. Contact e-mail: neuyangjian@163.com WENHONG LIU, Associate Professor, is with State Key Laboratory of Synthetical Automation for Process Industries, Northeastern University at Shenyang, Liaoning, P.R. China 110819. ZHENPING JI, Professor, is with the College of Information Science and Engineering, Shenyang Ligong University at Shenyang, P.R. China 110159. The online version of the original article can be found under doi: 10.1007/s11663-014-0079-y. Article published online October 16, 2014.

Universal Secondary Cooling Structure for Round Blooms Continuous Casting of Steels in Various Diameters

Based on the thermodynamic calculations, the universal secondary cooling structure, and the specific water rate for round blooms continuous casting in various diameters were investigated with a 2D heat transfer model. Results indicated that extension of the secondary cooling zone along casting direction significantly lowered the surface temperature recovery after the water cooling, though little influence on the recovering rate. The universal secondary cooling structure for the continuous casting of round blooms in diameters from 280 to 350 mm was optimized by appropriately extending the length of secondary cooling zone. The target unbending temperature was 908 °C for steel 16MnNb due to the thermodynamic calculation. To the optimized universal secondary cooling structure, the optimal specific water rates were 0.35 and 0.25 L kg−1 for the continuous casting of round blooms 280 and 350 mm in diameter, respectively. The present study had been applied to real production, resulting in round blooms in good quality, and contributing to the efficiency and flexibility of round blooms continuous casting process.

A Framework for Soft Sensing of Liquid Pool Length of Continuous Casting Round Blooms

Liquid pool length is a vital parameter for solidification control of continuous casting round bloom but it is difficult to be measured by direct hardware measurement. So in this paper, a framework based on heat transfer model for soft sensing of the liquid pool length has been presented. In the framework, the heat transfer model is the kernel and it has been calibrated for its machine-dependent parameters by solving the inverse heat transfer problem with the surface temperature measurements using a color pyrometer. The inverse heat transfer problem has been solved by the optimizer using chaos particle swarm optimization algorithm. After the calibration, the liquid pool lengths were predicted under different casting conditions. Finally, the predictions were validated by shell-thickness measurements using nail-shooting, as the measurements and calculations showed good agreement with the relative errors less than 1.5 pct. And the application of the framework for final electromagnetic stirring has also been presented.

CONTINOUSLY CAST BLOOM WITH INTERNAL DEFECT - FEM MODEL OPTIMIZATION

Continuously cast round blooms from vanadium microalloyed steel denoted as 25CrMo4 type are very sensitive to internal defect creation during casting. Further propagation of internal cracks is affected by suboptimal heating preceding the hot rolling process. FEM offers powerful tool for simulation of the temperature gradients and stress-strain behavior. In this paper it is demonstrated that proper selection of FEM model is essential to get trustworthy results corresponding with practical observations. It is possible to find optimum between two contradictory requirements – acceptable computation time severity and trustworthy results, if the simple model is compared with experimental data and/or more sophisticated models.

Stress-Strain Behaviour Simulation of Vanadium Microalloyed Steel with the Internal Defects During two Different Heating Strategies

This paper is focused on the explanation of a linkage between two different heating strategies and a grow of the internal casting defects, that are already present in continuously cast blooms made from vanadium micro-alloyed Cr-Mo steel. Internal defects initiation and propagation can be associated to unsuitable casting and heating conditions. Small casting voids surrounded by ferrite-bainite network enriched by carbides forming elements produce perfect conditions for crack grow during the bloom straightening, heating in the soaking pit and/or heat treatment. Moreover, we simulated the stress-strain behaviour of continuously cast round bloom during heating in soaking pit using the FEM software. It is shown that optimized heating strategy led to significantly lower rates of the plastic strain as a crucial value for the crack propagation.

Influence of Submerged Entry Nozzle on Intermixed Zone in Round Blooms with a Diameter of 525 mm / Wpływ Wylewu Zanurzeniowego Na Strefe Przejsciowa W Okragłych Wlewkach Ciagłych O Srednicy 525 mm

This work compares the experimental results of nickel concentration measurements in the intermixed zone of the continuously cast round blooms with a diameter of 525 mm using two types of submerged entry nozzles (SEN) - a straight-through nozzle and one with 5-ports. Based on determination of the system and optical interface in the blooms a detailed study of concentration profiles on the bloom surface in a small radius area, on the right side and then also on a cross-section of the blooms, was carried out. The results were further analysed using approximation models, and were to be used to verify the proposed model for predicting intermixed zones for a continuous casting machine, developed based on the results of physical and numerical modelling.

Numerical Simulation on Mold Electromagnetic Eccentric Stirring and Central Segregation of Round Bloom

The paper describes multi-section round bloom casting using external MEMS, equipped with max section D600mm and min D280mm mold, the center line of D280mm mold not coincident with the axis of stirrer coils. it is exist eccentric electromagnetic stirring of mold which section less than max D600mm, a mathematical model of MEMS has been established, the index of central segregation of D280mm macrostructure had decreased less than 1.12 by optimized parameters of electromagnetic stirring and SEN immerse depth, in the end, the quality of round bloom had improved.

Numerical Simulation of Fluid Flow in a Round Bloom Mold with In-Mold Rotary Electromagnetic Stirring

In this article, an electromagnetic field simulation and a flow analysis model are performed to describe the three-dimensional electromagnetic field distribution and the electromagnetically driven flow characteristics in a round-bloom mold with a low-frequency in-mold rotary electromagnetic stirrer. The interaction between the induced flow and the inertial impinging jet from a straight-through submerged entry nozzle (SEN) of the caster is considered. The effects of stirrer current and frequency on the electromagnetic field and the flow in the round-bloom mold are investigated, and a strategy to optimize the stirring parameters is proposed. The results show that the distributions of magnetic flux density and electromagnetic force magnitude are nonuniform in a three-dimensional electromagnetic stirring (EMS) configuration. There exists a significant axial induced component of electromagnetic force. The flow in the in-mold EMS system is characterized by a dominant swirling movement at the transverse sections, coupled with the recirculating flows in the axial direction. An upper recirculation zone and a lower recirculation zone with the reverse melt flowing are found near the strand wall at the axial location close to the middle of the stirrer, and another recirculation zone is formed due to the interference of the induced flow with the jet from SEN. The meniscus surface has a swirl flow, and the meniscus level rises near the bloom strand wall and sinks around the SEN wall. All of these flow features are closely associated with metallurgical performances of the in-mold rotary stirrer. With the increase of stirring current and the decrease of frequency, the magnetic flux magnitude increases. There is an optimum frequency to obtain a peak of electromagnetic force magnitude and maximum tangential velocity. For a mold rotary EMS system, to determine the optimum stirring intensity, it is necessary to make a compromise between a larger tangential velocity and a relatively quiescent meniscus surface.

Minimum Mould Exit Shell Thickness of Round Blooms for Peritectic Steel Casting

The minimum mould exit shell thickness (MEST) is one of key factors to consider for continuous casting machine design and the strand cooling schemes. Based on the shell strength at elevated temperatures, a concept of shell integral average strength, from its very top surface to the solidus frontier, has been introduced to judge its bulging tendency under ferrostatic pressure. Considering the contribution to the shell stability, a mathematic model for the determination of the minimum MEST has been brought forward. It is shown that, the minimum MEST of the round bloom is decided by not only the cross section and the minimum roller spacing, but also the opening degree of the support roller and the support roller on the side. Taking a round bloom casting of plain carbon steel (grade Q235) as an example, there is the cross point with the curve of the minimum MEST by the minimum roller spacing and the minimum MEST by the fluting include angle of the support roller and the support roller on the side against the cross section, if its own geometry structure cannot hold the ferrostatic pressure, the minimum MEST on the left of the cross point can remain as the determined amount by the minimum roller spacing; but on the right, it is increased with the increasing width of cross section.

MEST of Round Blooms with Different Foot Roll Pitch Designing for Peritectic Steel Casting

The minimum mould exit shell thickness (MEST) is one of key factors to consider for continuous casting machine design. Based on the shell strength at elevated temperatures, a concept of shell integral average strength, from its very top surface to the solidus frontier, has been introduced to judge its bulging tendency under ferrostatic pressure. Considering the contribution for the foot roller designing, a mathematic model for the determination of the minimum MEST has been brought forward. It is shown that, when the minimal foot roll pitch is fixed, the minimum MEST is steady. But at the same time, when the MEST does not exceed the minimum value, the round bloom off-round would happen, it is worse for the round bloom than the bulging for the normal strand. And there is the cross point with the curve of the minimum MEST by the minimum roll pitch and the minimum MEST by the fluting include angle of the support roller and the lateral support roller against the cross section.

Study on Fluid Flow in a Large Round Bloom Continuous Casting Mold

The fluid flow in an 800 mm diameter bloom mold from two kinds of submerged entry nozzles (SEN) was investigated numerically with a mathematical model and experimentally in a water model. It was shown from the research results that the impacting depths measured in the water modeling experiments are in good agreement with the ones from the numerical calculation. The impacting depths obtained from the 4-port down 5° SEN are 140~220 mm at immersion depths from 100 to 180 mm, while those from 5-port up 5° SEN are 60~120 mm at immersion depths from 100 to 140 mm. There are an upper and a lower recirculating zone for these two SENs at the vertical section through a port center. Dimension of these two recirculating flow zones varies with the immersion depth of the SENs. At 100 mm immersion depth of the tow SENs the eye of the upper recirculating flow is not evident. The eyes of the upper recirculating flows for 5-port up 5° SEN are close to the SEN, while the ones for 4-port down 5° SEN are close to the mold wall. On the liquid surface in the mold, flow produced with the 4-port SEN creates 8 recirculating flow zones close to the SEN, while 10 recirculating flow zones close to the mold wall are formed with the 5-port SEN.

An Iterative Algorithm to Determine Hot Strand Pressures during Continuous Casting of Round Blooms

Ovality defect of round bloom castings is the haunting defect of demanding grades steel, which can be attributed to excessive mechanical stress imposed by the roll containment especially at the unbending region. A 2-D elastic-plastic FEM model has been developed for the quantitative analysis of the effects of hot strand pressures on the ovality deformation and the contact normal force distribution. It is shown that higher friction force and smaller reduction deformation can be expected through the adoption of grooved rolls for given hot strand pressures as compared with plain rolls. For the determination of whether the given hot strand pressures can meet the requirement of strand downslide control in the caster, the resistances generated in the mold, secondary cooling zone and unbending zone are analyzed. Accordingly, an iterative algorithm has been presented to modify the hot strand pressures for given caster. A set of modified hot strand pressures has been computationally determined, which has been proved to be safe in production for casting round bloom with diameter up to Φ400mm with better roundness and less roller mark.

Monitoring of radionuclides in carbon steel blooms produced by EAF process

Because natural and artificial isotopes in steel might originate from steel scrap or from the residue of the material that was used in the technological process, thus monitoring especially artificial radionuclides 60Co, 137Cs and 192Ir deserve special attention. The analysis by g-spectrometry has been applied to determine the presence of natural isotopes 40K, 226Ra, 232Th and 238U as well as of the artificial isotope 60Co, 137Cs and 192Ir and their activity in the produced steel round blooms in the Steel Mill of CMC Sisak d.o.o. At the same time the content of radionuclides in the other materials (ferroalloys, bauxite, fluorite, lime, coke, graphite electrodes, refractory blocks) used in the same steel making process was investigated. The measured values regarding the presence of individual isotopes and their activity in steel were as follows: 40K all values were less than 1.6 Bqkg-1; 232Th all activities values were less than 0.02 Bqkg-1; 226Ra all activities values were less than 0.01 Bqkg-1; 238U all activities values were less than 1.10 Bqkg-1; 60Co all activities values were less than 0.02 Bqkg-1; 192Ir all activities values were less than 0.02 Bqkg-1 and 137Cs all activities values were less than 0.30 Bqkg-1.

Effect of Feeding Modes of Molten Steel on the Mould Metallurgical Behavior for Round Bloom Casting

Three kinds of modes for molten steel delivery have been studied in the paper for better understanding of their mould metallurgical effects during round bloom casting. The melt flow, free surface fluctuation, temperature field and solidification behavior in the mould region have been numerically analyzed upon the respective adoption of conventional straight single SEN, quad-furcated SEN with outlets in radial direction and a new type of quad-furcated SEN with outlets at tangential direction. For the latter, a strong horizontal swirling flow has been observed along with the upper and lower recirculation region in the mould. It is shown that the horizontal swirling flow in the mould can reduce the impingement depth of molten steel remarkably, inhibit the mould level fluctuation, and create an active bulk flow below the meniscus, which can also move the hot spot upward and promote superheat dissipation of the molten steel. Meanwhile, the temperature of molten steel near the free surface can be increased by 2.6 K to 4.4 K as compared with the two other normal nozzles. Moreover, compared with quad-furcated SEN with outlets in radial direction, the jet impingement on shell is much weaker, while the thickness of solidified shell at the cross-section of the mould is more even. Furthermore, the shell thickness at exit of the mould under the new type of SEN can reach approximately 18.6 mm, while that under quad-furcated SEN with outlets at radial direction is about 17.4 mm, which leave much allowance for the speed enhancement of present round bloom castings.