Bloom Continuous Casting Research Articles

Influence of cooling rate on secondary phase precipitation and proeutectoid phase transformation of micro-alloyed steel containing vanadium

During continuous casting process of low carbon micro-alloyed steel containing vanadium, the evolution of strand surface microstructure and the precipitation of secondary phase particles (mainly V(C, N)) are significantly influenced by cooling rate. In this paper, influence of cooling rate on the precipitation behavior of proeutectoid α-ferrite at the γ-austenite grain boundary and in the steel matrix are in situ observed and analyzed through high temperature confocal laser scanning microscopy. The relationship between cooling rate and precipitation of V(C, N) from steel continuous casting bloom surface microstructure is further studied by scanning electron microscopy and electron dispersive spectrometer. Relative results have shown the effect of V(C, N) precipitation on α-ferrite phase transformation is mainly revealed in two aspects: (i) Precipitated V(C, N) particles act as inoculant particles to promote proeutectoid ferrite nucleation. (ii) Local carbon concentration along the γ-austenite grain boundaries is decreased with the precipitation of V(C, N), which in turn promotes α-ferrite precipitation.

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 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.

Study on the reduction efficiency of soft reduction on continuous casting bloom

This paper explores factors influencing the reduction efficiency of bloom during soft reduction. The research results show that the center solid fraction affects greatly on the reduction efficiency of bloom, while the reduction amount, the width and the thickness of casting strand and the diameter of roller have little effect. The results from the comparison of the crown rollers and normal rollers show that the reduction efficiency of normal roller is larger than that of crown roller with 1 mm of reduction amount, while when the reduction amount is 5−15 mm, the reduction efficiency of normal roller is smaller than that of crown roller. However, the load borne by crown roller is much smaller than that of normal roller.

Technological intervention for continuously cast billet of martensitic stainless ultra high strength steel

Substantial market exists for martensitic stainless Ultra High Strength Steel (UHSS) with 12–13 wt% Cr due to its high hardness and strength, moderate temperature tolerance and corrosion resistance. However, columnar crystals, center porosity, shrinkage cavity and surface quality of martensitic stainless UHSS depend on special continuous casting (CC) technology parameters. In order to control the quality of billets, an industrial plant trial for optimizing the process parameters in a square bloom continuous casting was performed, in which influences of caster parameters (such as casting speed, temperature, electromagnetic stirring (EMS), mold flux, secondary cooling) on the surface quality and solidification macrostructure of martensitic stainless UHSS were investigated. The results indicated that an excellent surface quality and a good billet solidification structure of martensitic stainless steel could be obtained with appropriate measures, in which all of subsurface crack and depression have disappeared, and the equiaxial crystals in the center of the martensitic stainless UHSS billet was up to 50%, even to the maximum of 57%, the central porosity was below level 1.5 and more than 90% of the central shrinkage cavities have reached to level 1.0.

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.

RESEARCH ON INTERNAL CRACK SUSCEPTIBILITY OF CONTINUOUS-CASTING BLOOM BASED ON MICRO-SEGREGATION MODEL

The solidification and cooling of liquid steel in continuous casting process is a complicated nonequilibrium phenomenon.During steel solidification process,the micro-segregation of solute elements between liquid steel and solidified shell will vary with their temperature-dependent diffusion coefficients and equilibrium distribution coefficients.Due to non-uniform cooling pattern in the continuous casting process of steel blooms,the fluctuation of cooling rate in bloom will have a great influence on micro-segregation degree of the elements.The micro-segregation behavior of solute elements in steel solidification process is responsible for the variation of characteristic temperatures such as zero strength temperature(ZST),zero ductility temperature(ZDT) and liquid impenetrable temperature(LIT),which make up the brittle temperature range in steel solidification.During continuous casting process of steel,internal cracks created by thermal and mechanical deformation tend to occur in this range.To prevent the occurrence of these cracks in continuous casting bloom,it is essential to better understand about the internal crack susceptibility concerning micro-segregation behavior in the non-uniform cooling process.In this work,a micro-segregation analytical model for YQ450NQR1 steel continuous casting bloom is established to studythe inter-dendritic segregation behavior of main solute elements C,Si,Mn,P and S at various cooling rates,the results show that P and S are more likely to segregate compared with C,Si and Mn and the increase of cooling rate weakens the micro-segregation degree of C,Si,Mn,P and S.Based on the micro-segregation model established above,ZST,ZDT and LIT for YQ450NQR1 steel are calculated and the influences of cooling rate on ZST,ZDT and LIT are analyzed.It reveals that ZST,ZDT and LIT of YQ450NQR1 steel bloom decrease accordingly with the increase of cooling rate.On this basis,the index of internal crack susceptibility(IICS) is defined to quantitatively characterize the internal crack susceptibility of the bloom.The results show that the internal crack susceptibility becomes larger while the IICS value approaches to 1.Furthermore,an internal crack susceptibility model is obtained concerning IICS and cooling rate(CR) and the validation is performed to certify the model′s suitability in quantitatively predicting internal crack susceptibility of YQ450NQR1 steel continuous casting bloom in the non-uniform cooling process.

Numerical simulation of fluid flow and solidification in bloom continuous casting mould with electromagnetic stirring

A three-dimensional mathematical model of bloom continuous casting with mould electromagnetic stirring (M-EMS) has been developed. Stirring current effects on induced flow characteristics, heat transfer and solidification were investigated numerically. The results indicate that the electromagnetic force has a circumferential distribution on the plane transverse to the bloom strand, with maxima at the bloom corners. Joule heating produced by M-EMS is very small and can be ignored in the heat transfer calculations. Under the influence of rotational M-EMS, the swirl flow velocity of the melt decreases significantly when solidification is considered. With increasing stirring current, swirl flow intensifies remarkably, which prevents the superheated jet from the submerged entry nozzle flowing downwards and thereby reduces jet penetration into the liquid core. This increases the temperature of the melt in the mould and the temperature gradient at the solidification front, promoting superheat dissipation in the mould and enhancing the percentage of equiaxed zone in the strand.

Analysis and Application of Soft Reduction Amount for Bloom Continuous Casting Process

Based on the principle of solidification shrinkage compensation, a soft reduction amount calculation method was derived for bloom continuous casting process, and the bearing steel GCr15 was chosen as specific research steel to describe calculation process in detail. A two-dimensional heat transfer model was built to predict the solidification process of bloom, and the material properties of GCr15 were derived by weighted averaging of the phase fractions. The predicted temperature and shell thickness were verified by a thermal infrared camera and nail shooting results, respectively. The soft reduction amount of typical high carbon alloy steel blooms were calculated and discussed. The plant results showed that after the application of soft reduction to the bloom, centerline segregation and “V” type segregation were improved significantly. The carbon and sulfur ratios of the bloom centerline were reduced from 1.39 to 1.09 and 2.14 to 1.29, respectively.

Study on the Macrosegregation Behavior for the Bloom Continuous Casting: Model Development and Validation

With the aid of a coupling electromagnetic-thermal-solute transportation model validated by the industrial investigation, a three-dimensional (3-D) plus two-dimensional (2-D) hybrid modeling method has been presented for the exploration of the macrosegregation and macroscale transport phenomena in the bloom continuous casting (CC) processes of high-carbon GCr15-bearing steel. The evolution and characteristics of solute distribution and its influence on the porosity formation in the strand during CC process have been revealed. Solute segregation degree changes from a positive to a negative value with distance from strand surface in the region of initial solidification shell within thickness of 25 mm, which can be attributed to the circulation flow ahead the solidification front and the floatation of solute-rich molten steel at the upper part of the mold. The discontinuous, nonfrozen band induced by the zigzag solute distribution is proven to be the main reason that leads to the porosity formation in the final solidification stage of the CC strand. As the solidification proceeds, the segregation degree of C at the strand center is increased from 1.0 to 1.2, while the melt liquidus temperature is reduced from 1726 K (1453 °C) to 1706.91 K (1433.91 °C) during the CC process. Moreover, with the action of gravity and thermosolutal convection, a negative segregation region in the concave shape and an irregular positive segregation zone are produced in the fixed and loosened side of shell, respectively.

Optimization of CC Tail Bloom Operation

Cut-to-length bloom can not be controlled at the end of continuous casting process, and the yield of bloom is low. In order to improve the yield of bloom, the CC tail bloom system is proposed according to the bloom continuous casting conditions of equipment and process. Importantly, it is applied in the optimization of tail bloom operation in continuous casting process. Industrial tests show that the yield of bloom is improved obviously, with the quality of bloom is controlled as before.

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.

Formation of internal cracks during soft reduction in rectangular bloom continuous casting

To investigate the formation of internal cracks in GCr15 bearing steels during the soft reduction process in rectangular bloom continuous casting, fully coupled thermomechanical finite element models were developed using the commercial software MSC.MARC, and microstructures and fractographs were also observed. With the finite element models, the contours of temperature, equivalent plastic strain, and equivalent von Mises stress were simulated. It is observed that the fracture surfaces of internal cracks are covered by cleavage or quasi-cleavage facets. The region of internal cracks in the intergranular brittle fracture mode is in the mushy zone between the zero ductility temperature (ZDT) and the zero strength temperature (ZST). The simulated equivalent plastic strain in the crack region is 2.34%–2.45%, which is larger than the critical strain (0.4%–1.5%), and the equivalent von Mises stress is 1.84–5.05 MPa, which is within the range of critical stress (3.9–7.2 MPa), thus resulting in the occurrence of internal cracks. Reducing the soft reduction amount from 3 to 2 mm can lower the stress under the critical value.

Simulation Technique of AMESim Applied in Mould Electrohydraulic Servo Control System

Taking electrohydraulic servo control system used in mould oscillation in bloom continuous casting machine as the object of research, AMESim software is used to establish the model of the electrohydraulic servo control system ,also simulate and analysis the system,which theoretically confirms the reliability of the electrohydraulic servo control system.The software is also used for dynamic analysis about the control system.

Use of Simulation Techniques for the Design and Process Control of Continuous Casting Machines

This article presents the latest solutions implemented by Concast AG in the field of process simulation and illustrates the use of finite element method (FEM) models for the engineering of continuous casters and for process control in continuous casting. The first application presented is the use of FEM models for the design of the new Integrated Mould concept, which combines the benefits of both tube- and plate moulds for bloom casters. Second, the Concast real-time solidification model is presented. The article describes this model and its use for controlling the secondary cooling as well as to control mechanical soft reduction (MSR) as function of the actual casting parameters like temperature, casting speed, cooling intensity, both during steady and transient times. Field results show that this model permits us to improve the internal quality of continuous-cast blooms compared to static soft reduction. Finally, the development of a modular program for the design of continuous casters is presented. This integrated program is based on FEM and CAD and permits a fast optimisation of the caster design variables by linking together geometry, solidification, and mechanical calculation of strand solidification, bulging, and support. This approach contributes to a faster and more effective definition of any continuous caster in the early phase of the project, which results in cost and time savings.

Characteristics of solute segregation in continuous casting bloom with dynamic soft reduction and determination of soft reduction zone

A mathematical model accounting for the discharge of solute enriched liquid from the mushy zone with soft reduction by one pair of pinch rolls was established, and the ratio of solute concentration in the mushy zone before and after soft reduction, named as solute segregation ratio, is proposed to describe the characteristics of solute segregation. The theoretical analysis of solute segregation ratio at different soft reduction positions shows that the start point of the soft reduction zone should not be greater than the minimum optimum soft reduction position and the end point of the soft reduction zone should not be less than the maximum optimum soft reduction position corresponding to minimum solute segregation ratio. The soft reduction industrial practices of steel YQ450NQR1 and steel 37Mn2 were carried out with the soft reduction parameters determined by the model, and results showed that the inner quality was improved.

Mathematical model of heat transfer for bloom continuous casting

A mathematical model for heat transfer during solidification in continuous casting of automobile steel, was established on researching under the influence of the solidifying process of bloom quality of CCM in the EAF steelmaking shop, at Shijiazhuang Iron and Steel Co. Ltd. Several steel grades were chosen to research, such as, 40Cr and 42CrMo. According to the results of the high temperature mechanical property tests of blooms, the respective temperature curves for controlling the solidification of different steels were acquired, and a simulating software was developed. The model was verified using two methods, which were bloom pinshooting and surface strand temperature measuring experiments. The model provided references for research on the solidifying process and optimization of a secondary cooling system for automobile steel. Moreover, it was already applied to real production. The calculated temperature distribution and solidification trend of blooms had offered a reliable theory for optimizing the solidifying process of blooms, increasing withdrawal speed, and improving bloom quality. Meanwhile, a new secondary cooling system was designed to optimize a secondary cooling water distribution, including choice and arrangements of nozzles, calculation of cooling water quantity, and so on.