Cast Bloom Research Articles

A Study of the Oscillation Marks’ Characteristics of Continuously Cast Incoloy Alloy 825 Blooms

A comprehensive experimental study of oscillation mark (OM) formation and its characteristics during the solidification of Incoloy alloy 825 in the continuous casting of blooms is investigated by plant trials and metallographic study. The experiments involved two heats with the same casting and mold conditions and sampling at different locations across the strand. The metallographic study combined macro/micro-examinations of OMs and segregation analysis of Cr, Mn, Mo, Ni, and Si by microprobe analysis. The results show that OMs have widely different characteristics, such as mark type, depth, segregation, and accompanying microstructure. Furthermore, the mark pitch can vary considerably even for the similar casting conditions, leading to different conditions for the marks’ formation in relation to the mold’s cyclic movement. Finally, a mechanism for the OM formation is discussed and proposed. Possible solutions for minimizing the observed defects by optimizing the mold conditions are suggested.

Simulation and Testing of a Modified Mould on UGITECH’s ESRR™

Since the end of 2007, UGITECH S.A. has been equipped with an ESRR™ plant provided by INTECO Special Melting Technologies GmbH. This technology uses a superheated slag to remelt electrodes, in a T-shaped mould equipped with an upper graphite current collector: the current is split into two separate paths. This feature makes the ESRR™ process specifically appropriate to remelt a main electrode into smaller products like billets, with a rather high productivity. Nevertheless, the unique need of UGITECH S.A. to remelt continuous casting blooms, while keeping their 205 × 205 mm section constant, implies quite a small section electrode compared to the normal ESRR™ practice. The initial mould configuration of the machine had to be modified to take better into account this specific demand. The ESRR™ plant is a complex process which combines many simultaneous phenomena—electromagnetic, heat transfer, fluid flow, etc.—with a strong coupling. The first numerical model of this process was built, and data taken from literature [2-15] helped to develop its first heat transfer simulation. Despite the necessary simplifications and work hypotheses, this approach has given these first important results: to confirm the presence of organized toroidal motions in the slag, to provide orders of magnitude for variables one cannot measure in situ (slag flow velocity, temperature distribution), and to compare the ESRR™ mould in its original configuration with a new design. Several industrial trials, performed in the target configuration and simulated with a modified current upper collector, have demonstrated obvious improvements both on the surface quality of the ingots (directly related to the temperature at the metal meniscus) and on the stability of the machine’s regulation (melting speed and electrode immersion). After this confirmation of the advantages of the new configuration, UGITECH S.A. finally placed an order to INTECO GmbH for a new design mould and an associated body.

Application of swirling flow nozzle and investigation of superheat dissipation casting for bloom continuous casing

A swirling flow nozzle (SFN) has been proposed and designed for bloom continuous casting based on the idea that the melt flow pattern in the mould region can be controlled by changing the jet direction from outlets of the submerged entry nozzle. The simulated and plant trial results show that, as compared to the conventional straight nozzle, the oversized shrinkage porosity along with centre cracking at the strand cross-section is removed, and the maximum segregation degree fluctuation range of solute element C is reduced from 0.17 to 0.05. The positive effect is attributed to the remarkable superheat dissipation effect of horizontal swirling flow generated by SFN, which is reduced by 10.6 K as compared to the normal nozzle. Moreover, the adoption of SFN can further enhance the metallurgical effect of in-mould electromagnetic stirring (M-EMS), where both better chemical homogeneity and soundness of bloom castings can be obtained by the combined adoption of SFN and M-EMS.

Numerical simulation on level fluctuation in bloom casting mold with electromagnetic stirring

Based on a 380mm × 280mm bloom caster mold, the level fluctuation of steel-slag interface in the mold was simulated by the VOF model of commercial software Fluent. The effects of current intensity and frequency of EMS (electromagnetic stirring) on the level fluctuation in the mold were studied. The results show that whether or not with EMS, the maximum level fluctuation site of the mold occurs in the vicinity of the submerged entry nozzle. Compared with casting without EMS, molten steel flows horizontally rotatably under the action of the electromagnetic force by electromagnetic stirring, so the impact depth of molten steel decreases, then the level fluctuation slightly reduces, and the maximum level fluctuation value in the wide direction and the narrow direction of the mold, reduce from 4.24mm and 4.14mm to 4.04mm and 3.73mm respectively. With increasing intensity and frequency of current, the mold level fluctuation rises and the distribution uniformity of the level fluctuating amplitude worsens. The maximum level fluctuation enlarges by 0.18mm with raising the current intensity from 450A to 550A, but it enlarges by 0.79mm with 600A current intensity. The maximum level fluctuation enlarges by 0.15mm with raising the current frequency from 1.5Hz to 2.0Hz, but it quickly enlarges by 0.78mm with 2.5Hz current frequency. When the current strength and frequency are not more than 550A and 2.0Hz, level fluctuations are 4.00mm or less, which can meet requirements for controlling the bloom surface quality.

Effect of Final EMS on Shrinkage Cavity of Bloom

Final EMS parameters in the continuous bloom casting have been optimized by experimental and simulated researches. Simulation results show that the temperature gradient in the molten melt can be decreased and simultaneous solidification can be obtained with the final EMS. When the current is 600 A and frequency is 6 Hz, the maximum temperature gap between the inside and the brim of the melt bottom is 1.79 °C, which is 49.3% of that without EMS. Processing parameters are rectified based on the research results, the number of the shrinkage cavities is limited and the maximum diameter of the cavity is decreased.

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.

The optimisation of the secondary cooling water distribution with improved genetic algorithm in continuous casting of steels

An improved genetic algorithm is presented for the water consumption of the secondary cooling zone based on the heat transfer model of the off-line bloom caster. This study is to control the existing cooling systems and the steel casting practises in order to produce steel with best possible quality. The fitness function of improved genetic algorithm is founded according to the metallurgical criteria. This algorithm coupled with heat transfer model and metallurgical criteria, added dynamic coding method and self-adapting mutation on the original genetic algorithm can increase water distribution adaptively and improve the process efficiency. The simulation results of T91 bloom show that the optimised distribution reduced by 2% of water consumption comparing to that of before optimisation. The maximum surface cooling rate and the rate of temperature rise reduced, and the equiaxed rate increases. The function is built for explaining the relationship between the casting speed and water distribution.

Comparison of Experimentally Measured Temperature Gradient and Finite-Element-Method Simulations for Two Continuously Cast Bloom Heating Strategies

This paper deals with the comparison of experimentally measured temperature gradients and finite-element-method (FEM) simulations of two heating strategies that were used for continuously cast bloom soaking. The temperature gradient between the bloom surface and center was measured by two thermocouples incorporated directly into the bloom. Scanning electron microscopy equipped by energy dispersive X-ray spectroscopy analysis, hot tensile tests, and interdendritic solidification software was used for modeling of steel thermophysical properties with respect to the alloying-elements macrosegregation. The model of the bloom was programmed in the Fortran language. The FEM software MARC/MENTAT 2012 was used for simulation of two heating strategies (plane strain formulation). The first heating model was fitted to the commonly used heating strategy when internal defects grew above the critical limit. The second heating model was a newly proposed strategy that consisted of slower heating up to 1073 K when the first warming-through period occurred. The FEM simulations included determinations of the temperature gradient, the equivalent of stress, the equivalent of elastic strain, the equivalent of plastic strain, and the equivalent of total strain. The simulation results were in good agreement with experimental observations. The new heating strategy based on the FEM simulations led to significantly lower occurrence of internal defects in hot-rolled billets that are used for cylinder production.

Import-Replacing Re-Engineering of the Drive of the Rollers in the Intermediate Roller Table of a Continuous Bloom Caster

Issues related to re-engineering of the intermediate roller table of a four-strand continuous bloom caster are discussed. To make the table more reliable and facilitate its maintenance, it is proposed that all of the rollers be relocated outside the hot zone of the mill. This can be done by using transmission shafts to connect the journals of the rollers with the output shafts of the table’s motor-reducers.

Reduction in macrosegregation on 380 mm×490 mm bloom caster equipped combination M+F-EMS by optimising casting speed

Based on the plant trials and the developed coupled mathematical model of electromagnetism, turbulent fluid flow, heat and solute transport, the effect of F-EMS position on the degree of bloom macrosegregation cast with combination stirring (M+F-EMS) has been evaluated and compared for medium carbon steel, 40Cr, and high carbon steel, GCr15. The optimum F-EMS position is mainly related to the solidification characteristic of the molten steel in the liquid pool that depends on the average natural convection velocity and solidification time during the final solidification stage. For a given steel grade and a given F-EMS position, the trend of centre porosity and shrinkage cavity variations is to first decrease and then increase with increasing casting speed. The optimal solidification ratio at the F-EMS centre is increased as the carbon content increases due to the increasing solidification time, where the optimal solidification ratios for GCr15 and 40Cr are 72 and 68% respectively.

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.

Effect of early stages of thermomechanical processing on inclusions in high carbon steel

The effect of heat treatment and thermomechanical processing on size, size distribution, distribution and chemistry of non-metallic inclusions in a continuously cast high carbon steel was investigated using a Gleeble hot compression simulator at temperatures of 1050 and 1150°C and strains of 0·3 and 0·7 at strain rate 1 s−1. The results showed that the total number of inclusions classified into three main types, namely, sulphide, oxide and duplex, was reduced after the deformation at high temperature. Inclusions of different size and chemistry were found being present in different locations of the bloom, which was related with microsegregation in the as cast bloom. This suggested that inclusions were influenced by solidification conditions and subsolidus homogenisation. Furthermore, the observed changes of chemistry of inclusions during heat treatment and thermomechanical processing at high temperature (∼1100°C) are discussed. These changes were substantiated by evidence of devitrification of oxide inclusions including different manganese silicates.

Study on the Soaking Condition of High Carbon Chromium Bearing Steels

In order to seek the proper soaking condition of a high carbon chromium bearing steel, 100Cr6, a new approach was investigated considering the diffusion of chromium atoms. Although it is true that the large carbides are bigger with the worse degree of center segregation of continuously cast blooms, the size of center segregation band in blooms has a more accurate relationship with the degree of center segregation. Therefore, on behalf of the large carbide size in the conventional method, the size of center segregation band in continuously cast blooms of the steel has been used for the new approach. As a result, the center segregation and large carbides in them were removed completely by the new soaking condition.

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.

On the Relationship between Inclusions and Pores, Part II: Dendritic Structure, Pressure Drop in the Liquid and Pore Precipitation <sup></sup>

There is a relationship between pores and inclusions. As hypothesis goes, inclusions have an affinity to gather inside the pores and form clusters. Focus of this paper is how solidified dendritic structure affects the pressure field in the liquid and pore precipitation in austenitic stainless steel. Steel sample is a continuously cast bloom. Temperature profile and width of the mushy zone of the strand is modeled according to a constant temperature at the strands surface. Thermal analysis has been performed with differential thermal analysis (DTA) and differential scanning calorimeter (DSC). Dendrite arm spacing (DAS) is measured with light optical microscopy (LOM) and scanning electron microscopy (SEM). DAS is represented as the weight average of the distance between parallel sets of primary dendrite stems. Pressure field is calculated based on Darcys law. Pore formation is described through segregation of the gas components and pressure field in the liquid.

New approach to the soaking condition of 100Cr6 high-carbon chromium bearing steel

To establish the soaking condition of high-carbon, chromium-bearing steel, a new approach was investigated considering the diffusion of chromium atoms. Since, in practice, equilibrium can never be attained during solidification, it is proposed that large carbides start to form at a temperature slightly higher than the eutectic point. Although it is true that the large carbides are bigger; with a worse degree of center segregation of continuously cast blooms, the size of the center-segregation band in blooms has a more accurate relationship with the degree of center segregation. Therefore, in consideration of the large carbide size resulting from the conventional method, the size of the center-segregation band in continuously-cast blooms of steel has been used for the new approach. As a result, center segregations, and the large carbides in them, were completely removed by the new soaking condition.

Technology and process optimisation of bloom casting of ultrahigh speed rail steel

Since 2008, rail steels have been manufactured at No. 1 Steelmaking works of Wuhan Iron and Steel Corporation (WISCO). Initially a high percentage of defective rails were produced resulting from excessive inclusions, centre porosity and carbon segregation. This paper summaries a research project to improve this performance between the steel plant personnel and Wuhan University of Science and Technology. Measures were implemented including adjusting the composition and performance of mould flux, adopting a four-hole submerged entry nozzle, reducing the specific spray water flow and lengthening the soft reduction position. These measures significantly improved rail quality. Ultrahigh speed rails produced by WISCO are widely applied on the 380 km h−1 Beijing–Shanghai and Wuhan–Guangzhou railways. More improvements should still be developed to obtain more competitive and reliable rails.

Simulation of heat transfer and strain behaviour in bloom casting mould

An optimum casting model was developed to simulate the effect of mould flux on bloom heat transfer and strain behaviour based on a 3D MiLE method, and the influence of casting speed and superheat on bloom heat transfer and lubrication were also investigated. The simulation results showed that solidified shell thickness growth conforms to a Square Root Law, and that the model predicted results are basically in agreement with previous data in the literature, and provide confidence in model. The bloom temperature distribution range in the corner area is smaller than that in the mid-face, and the corner regions form a high cracking risk zone. The hot tearing indicator and effective stress in the corner area are significantly greater than that in the mid-face, so the corner area is the dangerous zone of cracking; The mould flux lubrication in the bloom mid-face is better than for the bloom corner region, due to a higher shell temperature and a fluid slag; The increasing of casting speed can delay air gap formation of the bloom corner area, improving the lubrication conditions, but when the casting speed is changed, it is also necessary for the mould flux viscosity and crystallization temperature be changed also. Increasing the superheat has little influence on the completely solidified distance of liquid flux in the bloom corner area.

Macrosegregation Improvement by Swirling Flow Nozzle for Bloom Continuous Castings

Based on mathematical model coupling electromagnetism, fluid flow, heat transfer, and solute transport, the metallurgical performances of conventional straight nozzle, swirling flow nozzle (SFN), and M-EMS have been evaluated and compared. The soundness improvement of bloom castings has been investigated by casting tests of adopting the newly designed SFN. As compared to the normal nozzle, center porosity has been eliminated along with the popular center radial crack, and a better chemical homogeneity was obtained by employing the SFN accordingly, where the maximum segregation degree of C and S at the strand cross section is decreased from 1.28 to 1.02 and from 1.32 to 1.06, respectively. Combined with the results of numerical simulation, the positive effect obtained can be attributed to the remarkable superheat dissipation under the implementation of SFN, where, compared with the normal nozzle, the melt superheat degree at the mold exit is reduced by 15.5 K, 9.8 K, and 17.3 K (15.5 °C, 9.8 °C, and 17.3 °C) under the other three casting measures of SFN, normal nozzle with M-EMS, and SFN with M-EMS, respectively.

Mathematical Modeling and Industrial Experiment of Liquid Steel Flow in the Three Outlets Continuous Casting Bloom Tundish

Abstract The dynamic development of the continuous steel casting (CSC) process has resulted in the application of this technology to the casting of steel semi-finished products on a mass scale. In the CSC process, before the cooling and solidification of liquid steel commences, the liquid metal dynamically flows through the steelmaking ladle, the tundish and the mould. Therefore, the control of steel flow is the key to the correct process. One of the metallurgical device in which the control of steel flow hydrodynamics is of crucial importance is the tundish. The subject of investigation within the present study was a three-nozzle tundish designed for casting of blooms. The software program Ansys-Fluent R was employed for the analysis of tundish operation. For the verification of the correctness of obtained results, an industrial experiment was carried out. For modification of the hydrodynamic conditions within the working volume of the tundish, two flow control devices were proposed, namely: a dam and a dam with an overflow window. The outcome of performed computer simulations were liquid steel flow fields and residence time distribution curves.