Slab Quality Research Articles

Technology Development for Producing Hot-Rolled Sheet Steel from Continuous-Casted Slab

To improve the quality of continuous-casted slabs and to reduce the cost of the produced rolled steel by ₽258 per ton, a resource-saving technology and submerged casting nozzle with eccentric holes are proposed. The experimental industrial tests of this technology for low-carbon and low-alloyed steel show an increase in the strength and toughness of the obtained hot-rolled metal. The results of the economic calculations at casting of 1.5 million tons of slabs and production of hot-rolled steel from them are provided. The computations of the ultimate strength and relative elongation of sheet steel were performed using well-known equations. The physical simulation results of sheet rolling in the finishing group of NShS and subsequent cooling establish the laws of metal structure formation.

Effect of strip feeding into mold on fluid flow and heat transfer in continuous casting process

Steel strip feeding into the mold during continuous casting is known as an innovative technology. The newly applied technology is designed to further improve the slab quality. To analyze the complex phase change processes, molten sodium thiosulphate (Na2S2O3·5H2O) was used in the experimental investigation as a transparent analog for metallic alloys. Then, a numerical model incorporating fluid flow, heat transfer and phase change during strip feeding into the mold process was developed. The generalized enthalpy-based method was applied to describe the phase change behavior, and the porous media theory was used to model the blockage of fluid flow by the dendrites in the mushy zone between the strip and melt as well as the solidified shell and melt. The validated model was then used for the simulation of the real strip feeding into the mold process in an industrial scale. The whole shape of the strip under the effect of jet flow from the submerged entry nozzle (SEN) was presented. Results show that the strip will reach a pseudo-steady state after experiencing steel sheath formation, steel sheath melting and strip melting processes. When using the feeding method that is the strip narrow side toward the SEN in the present condition, the strip immersion length can reach 4.5 m below the meniscus and the slab centerline temperature can be decreased by 21 K to a maximum. When the strip feeding speed increased from 0.3 to 0.5 m/s, the minimum temperature of the centerline could be lowered by 4 K or so.

Application of k-means clustering for temperature timing characteristics in breakout prediction during continuous casting

Breakout is the most serious incident in continuous casting. The missing and false alarms will seriously damage the caster and greatly affect the quality of slabs. In view of the defects of existing breakout prediction methods, k-means clustering and dynamic time warping (DTW) are combined to investigate and develop an effectual prediction method. Through extracting the typical temperature timing characteristics from the temporal and spatial perspectives, the method based on k-means clustering and DTW is proposed to distinguish and recognize the breakout. Compared with in-service breakout prediction system (BPS), the prediction results of the proposed method confirm that the number of false alarms can be reduced from 50 to 8 while ensuring a 100% correct alarm rate. The excellent prediction performance demonstrates that the clustering-based breakout prediction method exhibits good application potential, while it offers a novel approach to monitoring abnormalities in the continuous casting process.

Phase Transformation Modeling for Hypo Peritectic Steel in Continuous Cooling

Phase change of steel during cooling affects the slab qualities in continuous casting. Especially, crack susceptibility of hypo peritectic steel is high because large volume shrinkage occurs by peritectic phase transformation during solidification and cooling. In continuous cooling, phase change is different from the behaviors under the equilibrium condition, such as undercooling and extend of peritectic reaction, etc. Therefore, we develop a new phase change model considering thermodynamics, empirical equations, and carbon diffusion in each phase to predict phase change behavior during continuous cooling. In this model, phase change of hypo peritectic steel comprises 5 stages until all phases become the γ phase. The velocities of the δ/γ interface and phase fractions during cooling are calculated according to cooling rate, undercooling of the γ phase, and carbon contents. The results show that if solidification ends by the δ phase during dTp, the γ phase is formed by massive transformation. On the contrary, if peritectic reaction starts with liquid, the γ phase is formed and grows by diffusional transformation. In latter case, massive transformation of remaining δ phase can occur with high undercooling or very fast cooling rates. This analysis shows that there are several different paths depending on carbon contents of hypo peritectic steels. Phase change of hypo peritectic steel.

Investigation of Non-Sinusoidal Oscillator Driven by Double Servomotors for Continuous Casting Mold

To improve the surface quality and casting speed of the slab further, a new type mold oscillator synchronously driven by double servomotors was proposed in this paper. The experimental prototype was designed and manufactured. For the non-sinusoidal oscillator, the waveform modification ratio could be adjusted during the steel continuous casing operation. Firstly, the working principle of the oscillator was described. Secondly, the rotating rules of the servomotor to realize non-sinusoidal oscillation waveform for entire function were given. Finally, the laboratory experiments were made. The experiment results show that the error of the two-side oscillation curves is smaller and the actual technological parameters are close to the theory and the oscillator can realize non-sinusoidal oscillation well. This kind of non-sinusoidal oscillator provides a feasible scheme for reducing oscillator investment, increasing casting speed and enhancing slab quality.

실험계획법을 이용한 후판 압연기의 채터 진동 분석

Chatter vibrations have degraded rolling performances, influencing slab quality, rolling mill performance, and service life. However, chatter vibrations occur under various specific rolling conditions, so its analysis through experimentation is limited. In this study, while chattering occurs vibration data and multibody dynamics are employed to investigate the cause of chatter vibration of a thick plate rolling mill. As a result, it was confirmed that low-frequency chatter vibration is generated during the slab insertion process. The chatter vibration analysis was performed under various rolling conditions to reduce chatter vibration. According to the analysis results, the main design, interaction effect, and optimized rolling conditions were presented using an experimental design.

Three-Dimensional Distributions of Large-Sized Inclusions in the Surface Layer of IF Steel Slabs

The distribution of surface macro-inclusions is an important parameter that can directly influence the surface quality of IF steel sheets. In the present work, macro-inclusions > 100 μm within a 20-mm zone from the slab surface across the whole slab width were characterized by step machining methods, and the total analyzed area was 3,300,000 mm2. Three kinds of macro-inclusions were detected: bubbles (including single and aggregated), alumina associated with bubbles and refractory-related alumina. The three-dimensional distribution of surface macro-inclusions across the whole slab width was reconstructed, which showed macro-inclusions along the thickness direction almost concentrated 8 to 20 mm from the slab surface, corresponding to the center of the upper roll zone and stagnant zone below the submerged entry nozzle bottom according to the full-scale water mold model simulation. An inclusion stability model was established that indicated that increasing the flow velocity sharply decreased the stability degree at the solidification front because of the washing effect. The calculated results by this model agree with the fact that macro-inclusions were mainly concentrated in the slab center because of the low flow velocity at this location. The present work indicates that increasing the flow velocity at the solidification front as well as eliminating the stagnant zone is a potential way to improve the surface quality of IF steel slabs.

Mold-level prediction based on long short-term memory model and multi-mode decomposition with mutual information entropy

The mold is referred to as the heart of the continuous casting machine. Mold-level control is one of the keys to ensuring the quality of a high-efficiency continuous casting slab. This article addresses the failure of the mold-level prediction model in the actual production process to overcome the impact of noise. To improve the accuracy of mold-level prediction, a novel method for mold-level prediction based on the multi-mode decomposition method and the long short-term memory model is proposed. First, empirical mode decomposition of the mold-level data is performed. The actual eigenmode component number K is obtained through the calculation of the mutual information entropy of the eigenmode components. Then, we perform a K-based variational mode decomposition on the mold-level data. The noise dominant component is denoised by the calculation of the mutual information entropy of the eigenmode components. Moreover, the long short-term memory model is used to predict the noise dominant component and the information dominant component after denoising. Finally, the predicted result is subjected to variational mode decomposition reconstruction to obtain the predicted mold-level data. The experimental results show that compared with the other methods tested, the model has better prediction efficiency, prediction accuracy, and generalization ability. It provides a new idea for mold-liquid-level prediction and continuous casting blank quality assurance.

3D Center Segregation Reconstruction of Steel Continuous Casting Slab

The traditional detection method of the center segregation is limited to quantitatively evaluate the quality of steel casting slab, as it is observed in a 2D plane. Herein, a 3D reconstruction method of center segregation in steel slab is proposed. First, the ultrasonic scanning technology is used to scan the faults of layered and hot‐acid‐etched steel samples to obtain the sequence ultrasonic images. Second, the structures of segregation defects in each slice are segmented by an iterative threshold method. Then, image distortion is corrected and aligned to the same physical space. Finally, the 3D structure of segregation defects is reconstructed. A 70 mm × 30 mm × 2 mm sample of steel casting slab, which grinded with each 0.5 mm, is tested for evaluation, and experimental results show that the proposed method can accurately extract the structure of segregation defects. Based on the proposed method, its structure in 3D space is not only directly observed but also the volume (29.32 mm3) and surface area (649.48 mm2) of defects are calculated to measure the quality of steel slab more accurately. Therefore, it provides a new way to evaluate the steel‐slab quality.

Determination of solidification end position of continues cast slab based on force model

For a better determination of solidification end position during the process of soft reduction to confirm the reasonable reduction position and reduction area and to get the better slab quality, by analyzing the force behaviour of slab shell during soft reduction process, a force dynamic determination model for the end position of liquid core of slab during soft reduction process has been built. The variation trend of reaction force and total pressure of segment clamping-roller with the changing of position of solidification end can be got by the using of model calculation, and by analyzing this trend, a force feedback-based model has been proposed for checking the end position of liquid core. The veracity of the force feedback-based model has been verified by using nail-shooting approach.

Microstructure and Properties of Ni-Co Composite Cladding Coating on Mould Copper Plate.

In this study, a Ni–Co composite coating was fabricated successively on a copper substrate by laser, and the microstructure and properties of the composite coating were studied. Our results show that the elements from Ni and Co alloy powder were uniformly distributed in the binding region, which obtained a good metallurgical bonding with the substrate. The microhardness of Co-based coatings was 646 HV0.1, which was approximately 7.6 times greater than the hardness of Cu substrate. The Ni-based coating hardness was a little lower than that of the Co-based coating, which was approximately 7.0 times higher than the hardness of Cu substrate. The Co-based coating was comprised of reinforced phases, such as Co, Ni–Cr–Co–Mo, M7C3 and {Fe, Ni}. The wear volume of Co-based coatings was approximately 0.164 mm3 after the sliding wear test at 60 min, which was only 4% of the substrate’s wear volume. Meanwhile, the heat shock resistance experiment was carried out at 800 °C for 75 times before the Ni–Co composite coating was peeled off. It was shown that this Ni–Co composite coating had good wear resistance and high temperature properties, which is helpful to improve the service life of a mould copper plate. It not only saves the economic cost of steel plants, but also improves the quality of slabs and products.

Integrated Approach to Density-Based Spatial Clustering of Applications with Noise and Dynamic Time Warping for Breakout Prediction in Slab Continuous Casting

Mold breakout is a catastrophic accident that has serious impacts on smooth production, slab quality, and caster equipment. Accurate identification and prediction of an impending breakout are always top priorities in continuous casting operations. In view of crucial common features of mold copper plate temperatures during a breakout, such as time lag and space inversion, the concepts of density-based spatial clustering of applications with noise and dynamic time warping are introduced, and an integrated novel method for breakout prediction is developed. Through extracting and fusing the representative singularity and approximation of temperature variation, the typical temporal and spatial temperature characteristics during breakout can be distinguished and predicted accurately. Compared with traditional methods of logical judgment and artificial neural network, the method based on clustering does not need to modify forecast thresholds or parameters artificially, which overcomes the limitation of model dependence on human beings, and demonstrates excellent adaptability and robustness for online abnormality prevention.

Dynamic optimization method of secondary cooling water quantity in continuous casting based on three-dimensional transient nonlinear convective heat transfer equation

The defects of a slab, such as shrinkage and crack, are related to non-uniform cooling in secondary cooling zone (SCZ) of continuous casting, hence setting the value of water flow rate in the SCZ plays a vital role for the quality of slab. The traditional scheme can lead to the great fluctuation of temperature when the casting speed changes. Therefore, based on 3-dimensional transient nonlinear convective heat transfer equation of a slab, this paper develops a dynamic optimization method for setting the value of water flow rate. However, it is difficult to realize this method due to the following two causes. 1. It costs too much time to calculate the 3-dimensional transient nonlinear convective heat transfer equation. 2. It is not easy to select a suitable step size for the optimization algorithm. So this paper presents a modified adaptive step size quasi-Newton parallel iterative algorithm, which can not only reduce the computing time but also select a suitable step size. Actual data in a steel plant are used to demonstrate its validity, and the results clearly show that this scheme can greatly reduce the running time and give a more stable temperature distribution.

Fabrication, microstructure and optical properties of large-sized Nd:YAG and composite Yb:YAG transparent ceramic slabs

Abstract Large-sized Nd:YAG and composite Yb:YAG transparent ceramic slabs of high quality were fabricated by the conventional solid-state reaction sintering technique. The Yb:YAG transparent ceramic slabs were surrounded by undoped YAG transparent ceramics and this composite structure was built in the process of ceramic shaping. The difference in microstructure between Yb:YAG and YAG transparent ceramics was not observed and the composite Yb:YAG slabs showed fully dense and pore-free microstructure. The in-line transmittances at 400 nm reached 81.7% and 79.7 % for YAG and Yb:YAG transparent ceramics, respectively, indicating good optical quality of the composite ceramic slab. And the optical transmission micrograph was very homogeneous with only a few isolated scattering centers. These results suggested that building composite structure in the process of ceramic shaping was a simple and reliable technical scheme. Moreover, the large-sized Nd:YAG transparent ceramic slab with size of 150.0 mm × 50.0 mm × 4.0 mm was also prepared. The calculated absorption coefficient was about 0.002 cm-1 @1064 nm, which suggested that the optical quality of this Nd:YAG transparent ceramic slab had reached the level of Nd:YAG crystals.

Mold-Level Prediction for Continuous Casting Using VMD–SVR

In the continuous-casting process, mold-level control is one of the most important factors that ensures the quality of high-efficiency continuous casting slabs. In traditional mold-level prediction control, the mold-level prediction accuracy is low, and the calculation cost is high. In order to improve the prediction accuracy for mold-level prediction, an adaptive hybrid prediction algorithm is proposed. This new algorithm is the combination of empirical mode decomposition (EMD), variational mode decomposition (VMD), and support vector regression (SVR), and it effectively overcomes the impact of noise on the original signal. Firstly, the intrinsic mode functions (IMFs) of the mold-level signal are obtained by the adaptive EMD, and the key parameter of the VMD is obtained by the correlation analysis between the IMFs. VMD is performed based on the key parameter to obtain several IMFs, and the noise IMFs are denoised by wavelet threshold denoising (WTD). Then, SVR is used to predict each denoised component to obtain the predicted IMF. Finally, the predicted mold-level signal is reconstructed by the predicted IMFs. In addition, compared with WTD–SVR and EMD–SVR, VMD–SVR has a competitive advantage against the above three methods in terms of robustness. This new method provides a new idea for mold-level prediction.

Analysis of the Influence of Segmented Rollers on Slab Bulge Deformation

The bulge deformation of the continuous casting slab must be controlled in order to improve the slab quality. In this study, a coupled three-dimensional thermomechanical model is suggested based on dynamic contact between the slab and the rollers, so as to investigate the influence of the rollers in reducing slab bulge deformation. Moreover, the rigid casting rollers in this model are replaced by elastic casting rollers in order to improve the calculation accuracy. Further, the influence of two-segment and three-segment rollers on the slab bulge deformation is systematically studied. The results indicate that the bulge deformation of the slab increased by 74.3% when elastic casting rollers were adopted instead of rigid casting rollers. This deformation was reduced by 29.7% when three-segment rollers were used instead of two-segment rollers. Moreover, the influence of the roller spacing and the roller diameter of the segmented roller on the deformation was studied in detail. In order to achieve the purpose of controlling the bulge deformation, improved segmented roller spacing and diameter were proposed, leading to a 75.4% reduction in the bulge deformation.

A Simulation Study on the Flow Behavior of Liquid Steel in Tundish with Annular Argon Blowing in the Upper Nozzle

A three-dimensional mathematical model of gas−liquid two-phase flow has been established to study the flow behavior of liquid steel in the tundish. The effect of the argon flow rate and casting speed on the flow behavior of liquid steel, as well as the migration behavior of argon bubbles, was investigated. The results from the mathematical model were found to be consistent with those from the tundish water model. There were some swirl flows around the stopper when the annular argon blowing process was adopted; the flow of liquid steel near the liquid surface was active around the stopper. With increased argon flow rate, the vortex range and intensity around the stopper gradually increased, and the vertical flow velocity of the liquid steel in the vicinity of the stopper increased; the argon volume flow in the tundish and mold all increased. With increased casting speed, the vortex range and intensity around the stopper gradually decreased, the peak value of vertical flow velocity of liquid steel at the vicinity of the stopper decreased, and the distribution and ratio of argon volume flow between the tundish and the mold decreased. To avoid slag entrapment and purify the liquid steel, the argon flow rate should not be more than 3 L·min−1. These results provide a theoretical basis to optimize the parameters of the annular argon blowing at the upper nozzle and improve the slab quality.

Review of non-reactive CaO–Al2O3-based mold fluxes for casting high-aluminum steel

Advanced high-strength steels (AHSSs) have been gradually applied to modern auto industry, as they have the advantages of improving the steel strength and lightening the car weight, which not only ensures the safety but also saves the energy. However, the high-aluminum (Al) content in AHSSs may react with SiO2 in conventional CaO–SiO2-based mold flux during the process of continuous casting, which leads to the deterioration of the mold flux properties and a poor slab quality. Then, the non-reactive CaO–Al2O3-based mold flux was proposed and has been developing for the casting process of high-Al steels, but there are some problems of low consumption and insufficient lubrication that need to be solved. Thus, previous researches on the effect of each component on the properties of CaO–Al2O3-based mold flux were systematically summarized, and the situation of plant trials on CaO–Al2O3-based mold flux was evaluated. The results indicated that the proposed CaO–Al2O3-based mold fluxes could avoid the slag-metal reaction problems; however, the problems of lubrication, crystallization and heat transfer issues still exist. Therefore, tremendous works still need to be conducted for the development of new generation of CaO–Al2O3-based mold flux system. The review was performed aiming to provide a technical guidance for designing and optimizing CaO–Al2O3-based mold flux system that meets the demand of the continuous casting process of high-Al steels.

Structure of Solidified Films of CaO-SiO2-Na2O Based Low-Fluorine Mold Flux

As an essential synthetic material used in the continuous casting of steels, mold fluxes improve the surface quality of steel slabs. In this study, a CaO-SiO2-Na2O-based low-fluorine mold flux was solidified by an improved water-cooled copper probe with different temperatures of molten flux and different probe immersion times. The heat flux through solid films and the film structures were calculated and inspected, respectively. Internal cracks (formed in the glassy layer of films during solidification) were observed. The formation and evolution of those cracks contributed to the unstable heat flux density. The roughness of the surface in contact with the water-cooled copper probe formed as films were still glassy and the roughness had no causal relationship with crystallization or devitrification. Combeite with columnar and faceted dendritic shapes were the main crystal in the film.

Control of Jet Flow Angle in Continuous Casting Process using an Electromagnetic Brake

Abstract The flow pattern in the mould of the continuous casting is an important factor in determining the quality of the steel slabs that are produced in the end of the process. Hence it can heavily influence manufacturing costs due to the scrap percentage. Electromagnetic actuators are frequently used in the continuous casting process to stabilize the flow in the mould and therefore produce higher quality of steel slabs. Usually they are used in open loop but their effect on the flow pattern may be much better directed if they are used as a part of closed loop control based on real time measurements. In this paper, a closed loop controller is proposed that adjusts the magnetic field of an electromagnetic brake using the real time measurement of the angle of the jet flowing from the Submerged Entry Nozzle (SEN). The angle is kept within a specific range by the controller in order to prevent a deeper jet impingement into the mould; this allows us to achieve the desirable double roll flow pattern, and to avoid the entrapment of slug. The controller is based on a model of the relationship between brake current and jet angle that was obtained using experimental data from a laboratory scale continuous casting plant.