Cast Bloom Research Articles

Summary of the Function of Sn in Iron and Steel

In the steel industry Sn is always regarded as harmful impurity element, one of "Five harmful elements", which affects the welding property and equality of steel product especially the equality of continuously cast bloom and causes hot brittleness, temper brittleness, crack and fracture. While Sn element also has a vital influence on free-cutting steel, electrical sheet, cast iron and stainless steel. NSSC developed ferritic stainless steel containing low Sn and Ni content improving the corrosion resistance performance and the workability significantly. So the Sn element, as cheap alternative adding element, can substitute for Cr and Ni. It's helpful to study the effect of tin in the iron and steel and develop new stainless steel products of high equality conserving chromium resources. In this way it will promote the variety and technique of stainless steel, ensure the independence on resources and conserve resources remarkably.

The Simulation Analysis on the Heat Transfer Model of Bloom

A mathematical heat transfer model was built to simulate temperature, shell growth and phase transformations in continuous casting based on the technical conditions of the bloom caster of some steelmaking plant. The simulation result is coincided with the measurement. The effects of the operation parameters including casting speed, bloom size and steel melt superheat on the solidification process were discussed and the means of enhancing the bloom temperature was brought forward. The research results have been applied in the practical manufacture process to compute the best casting speed, to control the flow rate of secondary cooling water and to optimize the spray pattern of secondary cooling zone. The bloom quality and productivity could be improved.

A new experimental method for identifying the conditions necessary for diffusion bonding in free cutting steels

Porosity in cast bloom steel may lead to reduced strength or inconsistency of final rolled bar products and components if not properly closed and healed during the hot rolling process. Partial or complete recovery of strength in such porous materials can be achieved by diffusive healing processes at elevated temperatures. Devising an appropriate healing process that does not cause discontinuity in the microstructure, and in the mechanical properties at the bonding sites, whilst preventing distortion of the component during bonding requires an accurate choice of thermo-mechanical processing parameters. Despite work carried out on optimising diffusion bonding in materials such as titanium alloys, aluminium alloys and copper, the diffusion bonding process in free cutting steels has received relatively little attention. To support thermomechanical process optimisation (e.g. rolling) and calibration of theoretical models, a new experimental method was developed to determine combinations of load, temperature and time sufficient for complete diffusion bonding in as-cast Free Cutting Steels (FCS). The extent of diffusive healing and bond strength were examined by tensile testing to failure corroborated by SEM examination of the bond line. This enabled optimal loading conditions for the formation of a complete, strong bond to be identified.

Modeling and Measurements of Intermixing Time in a Water Model of a Four Strand Steelmaking Tundish System

Intermixing of two different grades of steel in a 17 tonne, 4 strand bloom casting has been investigated in a ~0.3 scale, geometrically and dynamically similar, isothermal water model tundish system. In this, intermixing phenomena were studied by simulating mixing of flowing water in the tundish, having different dissolved tracer (electrolyte) concentrations. Electrical conductivity measurement technique was employed to monitor concentration of dissolved electrolyte near the strands as a function of time and thereby determine intermixing time. For each experimental condition, three measurements were made and based on such an average intermixing time was estimated. Reproducibility was found to be always within ±15 %. Influence of key operating variables such as, inflow rate from ladle, net outflow rate from tundish, residual liquid volume left over from the previous grade etc. on the duration over which intermixing occurs (referred to as in the text as the intermixing time) was investigated. It was found that any increase in residual volume as well as inflow rate tends to prolong intermixing time. In contrast, influence of outflow rate was quite the opposite. Furthermore, while variation of intermixing time among strands was only marginal, tundish interior design (viz., presence of flow modifiers, pouring box etc.) was found to have considerable influence on intermixing time. Flow phenomena (observed visually through the dispersion of KMnO4 solution) in the given tundish was found to be practically symmetrical about the transverse centre-line and so was the associated intermixing time. Embodying a large number of experimental data, explicit correlations for intermixing time were derived in terms of principal operating variables through dimensional analysis and regression. Two different versions of correlations, applicable respectively to a flat bottom as well as a wedge fitted tundish systems, were developed. In SI unit, these are respectively represented as: in which, Q in is the input flow rate, Q out,T is the net outflow rate, V res is the residual liquid volume and τ int.mix is the intermixing time corresponding to a degree of 95 % homogeneity. Finally, adequacy and appropriateness of the proposed correlations are assessed both from theoretical and experimental stand points.

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.

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 Modeling of the Fluid Flow in Continuous Casting Tundish with Different Control Devices

Numerical simulations were conducted to study the melt flow under the influence of control devices in a T-type two-strand bloom caster tundish via the open source Computational Fluid Dynamics software OpenFOAM. Three different cases were studied: a bare tundish, a tundish with two pairs of baffles, and a tundish equipped with a turbulence inhibitor and a pair of baffles. Turbulence inhibitor and baffles arrangement showed an improvement of the fluid flow characteristics, yielding lower values of dead volume and higher values of plug flow. With a turbulence inhibitor, the velocity of metal which flows directly toward the tundish floor is smaller and the turbulence kinetic energy of the melt top surface is lower than the other two arrangements.

Hybrid laser-EMAT system for non-destructive testing continuous casting billet

A novel laser-electro magnetic acoustic transducer (EMAT) system for nondestructive testing NDT surface crack of continuous casting billet (CCB) is provided. Rayleigh wave generated by line laser source is used to detect the surface crack of CCB. According to the principle of mode conversion from Rayleigh wave to shear wave, the defect signal is received using the shear wave EMAT sensor in a non-contact way. Experiments are carried out on the steel sample with size 30 × 0.2 × 0.2 (mm) of crack. Further, the influences of life off value and distance between EMAT sensor and laser beam on the testing sensitivity are discussed, respectively. It is found that the life off value is the main factor that effects sensitivity of the proposed method. There is a clear prospect of the method applied to test continuously cast bloom at high temperature.

On the Relationship Between Inclusions and Pores, Part I: Precipitation

Formation mechanism and clustering of TiN–Al2O3 inclusions precipitated during solidification of austenitic stainless steel is analyzed in this paper. Steel sample are taken from a continuously cast bloom. Clustering of inclusions was examined with a hypothesis that inclusions gather inside the pores. Precipitation of the inclusions occurs through segregation in the residual melt in the interdendritic area. Size and mean distance of the inclusion particles are calculated with mass balance. Impact fracture test in cryogenic temperature reveals clusters of inclusions inside pores on the fracture surface. Size and distribution of the inclusions are examined using light optical microscopy and scanning electron microscopy. Inclusion’s composition is investigated with energy dispersive X-ray spectroscopy analysis. Differential thermal analysis and differential scanning calorimetry Tests determined the transformation temperatures. Results of the experiments are in good agreement with theoretical analysis.

Thermodynamics Analysis of Non-Metallic Inclusions Formation in the Liquid Steel Flow Through Continuous Casting Tundish

Thermodynamics Analysis of Non-Metallic Inclusions Formation in the Liquid Steel Flow Through Continuous Casting TundishExperiments were conducted at industrial plant to determine the free and total oxygen contents in molten steel in the tundish during continuous casting blooms of sizes 280×280 mm. On the basis of industrial experiment results a thermodynamic evaluation of non metallic inclusion formation in liquid steel was performed. Software FactSage® with thermodynamic base packages were tested and applied to calculate equilibrium formation of oxides and sulphides. The results showed the effect of oxygen contents and temperature on the formation inclusion in liquid steel. Calculation results was presented in the form of suitable characteristics which were illustrated graphically.

A New Method to Determine Internal Reduction Efficiency during Continuous Cast Bloom under Soft Reduction

To clarify the efficiency of internal reduction during continuous casting bloom when the technology of soft reduction is applied, a thermal-mechanical coupled FEM model was developed to investigate the internal reduction efficiency. Basing on the model developed in this article, the area variation of the mushy zone under soft reduction could be calculated. A new calculation method of internal reduction efficiency through the area variation of mushy zone was proposed. The variation trend of internal reduction efficiency as a function of soft reduction amount, calculated by two different methods, was introduced.

Effect of the Roll Surface Profile on Centerline Segregation in Soft Reduction Process

This study investigates the effect of roll surface profile on the centerline segregation of the molten metal (bloom) being solidified in the soft reduction process, where the molten metal (bloom) thickness is reduced consecutively by a set of rolls. The roll surface profiles are convex and flat. Finite element analysis was performed to examine the variations in solidus line contour, the phase state across the bloom cross-section and stress state of the bloom in the mushy zone to the casting direction when high carbon bloom (400 mm in thickness and 500 mm in width) was solidified and plastically deformed at the same time. To verify the finite element analysis results, soft reduction test with an actual bloom caster was also carried out. Results reveal that a convex roll surface profile is much more effective than a flat roll surface profile in decreasing the centerline segregation and the internal cracks in the bloom. The convex roll surface profile compresses the mushy zone from both directions (bloom thickness and width) and subsequently compensates efficiently the solidification shrinkage of molten metal. In addition, the convex roll can save energy required in deforming the bloom in the soft reduction process.

Features of Deformation of Partly Crystallization Blooms at their Two-Stage Soft Reduction

Steady increase of requirements to quality an axial zone continuously cast blooms stimulated in last time development of principles of its deformation at the end of solidifications with the purpose of suppression of axial porosity and segregation: soft reduction or mechanical soft reduction. The technology of soft reduction is one of the most effective ways of improvement quality of internal layers continuously cast blooms. In this case the central part of section of continuously cast blooms is in liquid or liquid-solid condition (mushy zone). At the same time, in practice now there are, at least, some original technical decisions for realization of this method.

Reduction of Tundish Skull and Yield Improvement in Steel Plants Through Physical Modeling of Steelmaking Tundish Systems

New types of furniture (also termed as flow modifiers or baffles) were incorporated in industrial scale, slab and bloom casting tundish systems with an aim to reduce residual metal loss (i.e., tundish skull) at the end of sequence casting. To this end, water model experiments were carried out in which, slag vortexing phenomena during emptying of tundish was studied embodying different types of furniture into existing tundish designs. These in general indicate that a wedge shaped bottom together with an embedded pouring box applied in conjunction have the potential to reduce tundish skull and improve yield losses significantly. In addition, limited residence time distribution measurement experiments were made to investigate metallurgical performance of modified design tundish systems. These indicate that deployment of new furniture with minor design modifications, despite contributing to a reduction in tundish capacity (10–12%), do not influence metallurgical performance of steelmaking tundish systems to any significant extent. Accordingly, designs of currently employed slab (32 and 37 tonnes capacity respectively) and bloom casting tundish (10 and a 17 tonnes capacity respectively) systems were modified in four different steel mills and plant trials conducted to assess the extent of yield improvement. Significant improvements in yield losses, to the extent of 50–60%, have been confirmed by the industry during sequence casting.

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.

Influence of Casting Speed on Solidification Process and Solidification Structure of Continuously Cast Bloom

On the basis of problems about quality of steel products that a certain special steel factory facing, a heat transfer model about continuous casting process was first established, then the influence of casting speed on solidification process and solidification structure of continuously cast bloom was comprehensively studied. It is shown that shell thickness leaving mold, crater length and the value of H(which is chosen as the measurement criterion of the number of the heterogeneous nucleation nucleus) are influenced most with increasing casting speed. Meantime, compared with proportional control method, solidification process of continuously cast bloom is in uniform variation with small fluctuation by target surface temperature method. What’s more, when centre solidification time is considered only, central zone macrosegregation may form more easily at last by proportional control method with increasing casting speed, but it is opposite by target surface temperature method; when the value of G/V1/2 and the value of H are considered only, the ratio of equiaxed grain will increase with increasing casting speed by proportional control method, but it is opposite by target surface temperature method.

Effect of melting and casting parameters on the hot ductility behavior of Nb-bearing beams, billets and slabs

The production of defect free continuously cast microalloy Nb-bearing steel slabs, blooms and billets have increased in importance for both the producer and the end user. The minimization of surface and internal defects has a substantial impact on steel producing operating costs, internal and external cost of quality and delivery performance. A keen and thorough understanding of the hot ductility behavior of various steel grades is essential to successfully melt and cast high quality microalloy steels. This paper describes current steelmaking and casting considerations and recommendations to successfully cast high quality niobium-bearing slabs, billets and blooms. This study also integrates the process metallurgy and the physical metallurgy to better understand the hot ductility mechanisms and process metallurgy control practices in use today for the high quality production of Nb-bearing steels.

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.

Model Study about the Influence of Superheat on Macrosegregation Formation in Continuously Cast Bloom of Special Steel

Aiming at the facing problems of the actual continuous caster in a certain steel mill, a solidification-process heat transfer model about continuously cast bloom of special steel was first established, then the influence of superheat on macrosegregation formation was studied. It is shown that the crater length will become longer, the shell thickness will become thinner with increasing the superheat. Meanwhile, when decreasing the superheat , the value of will decrease and the value of H will obviously increase, then the equiaxed crystal ratio will be raised greatly. However, the bloom centre solidification time will increase. Consequently, attention should be paid to this point when decreasing the superheat in order to lessen the extent of macrosegregation.

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.