Slab Continuous Casting Research Articles

Application of big roll and heavy reduction in continuous casting of thick slabs

ABSTRACT Porosity and segregation are common internal defects in thick slabs, which negatively affect the flaw detection qualification rate and mechanical properties of heavy plate materials. This study presents a novel big roll and heavy reduction (BRHR) technology to mitigate these internal defects. The Q345 slab with a thickness of 400 mm and a width of 2400 mm was selected as the specific research object. The cooling strategy was investigated to optimise the solidification front during the BRHR process. Achieving uniform solidification within the slab width significantly reduced porosity during the BRHR process. As the BRHR amount increased from 0 to 20 mm, porosity spots in the center of the thick slab significantly decreased. Additionally, carbon segregation decreased in slabs with BRHR amounts of 10 and 20 mm compared with 0 mm BRHR slabs. Statistical measurements revealed that as the BRHR amount increased from 0 to 20 mm, the ratio of porosity spot area decreased from 0.244% to 0.014%, while the maximum carbon segregation degree decreased from 1.17 to 1.09. Optimal control of uniform solidification across width direction and the total BRHR amount contributed to improve solidification porosity and segregation.

Prediction and optimisation of solidification structure growth and transformation behaviour in microalloy chamfered continuous casting slab

The solidification structure of complex chamfered slabs of microalloyed steel was analysed and controlled. A computational model of solidified structures was applied and the model was verified. The research results show that there are differences in the solidification structure inside the slab. The slab successively produces fine-grained areas, large-size columnar grain areas, CET (columnar to equiaxed grain transition), and equiaxed grain areas. The effects of different carbon contents on the solidification structure were analysed. As the c content decreases from 0.23% to 0.20%, the tip growth fitting coefficient a3 increases from 4.077172e-06 to 5.403274e-06, which increases the grain growth rate. Considering the different sensitivities of grain growth in the lateral direction, a lateral zoning water supply strategy is used to flexibly control the uniformity of grain growth. By simulating different water flow ratios in the corners and centre areas, the optimal nozzle water flow density ratio is 0.840. Superheat can effectively control grain distribution. The proportion of central equiaxed grains increases with decreasing superheat. The study found that the proportion of equiaxed grains has different sensitivities affected by different degrees of superheat. The superheat degree ranges of strong sensitivity and weak sensitivity were obtained. In the production practice of superheat adjustment to control particle distribution, the adjustment effect is better within the strong sensitivity range.

New route for fabricating low-carbon ductile martensite to optimize the stretch-flangeability of dual-phase steel

Traditional cold-rolling dual-phase steel (CR sheet), limited by poor stretch-flangeability as indicated by the hole expansion rate (HER), faces challenges in manufacturing complex automobile parts. To address this, a new type of hot-rolling dual-phase steel with an 800 MPa grade (manufactured by tyipical thin slab continuous casting and rolling process, TSCCR) was designed to replace the traditional one. The TSCCR sheet, with a lower carbon content of 0.047wt.%, compared to 0.093wt.% in the CR sheets, features 40.6% martensite significantly higher than 28.2% in the CR sheet. This increased martensite content, despite being softer and more ductile due to its low hardness, compensates for the tensile strength in the TSCCR sheet. Furthermore, the TSCCR sheet exhibits a unique heterostructure characterized by uneven carbon distribution from the ferrite/austenite interface to other boundaries/interfaces, estimated by the negligible partition local equilibrium (NPLE) model of ferrite transformation. This unique heterostructure diminishes the mechanical disparity between ferrite and martensite, resulting in a higher yield strength of 568 MPa, compared to 456 MPa in the CR sheet. During uniaxial tension, the ferrite and ductile martensite in the TSCCR sheet initiate cooperative plastic deformation earlier than in the CR sheet, despite initially having fewer favorable slip systems in the TSCCR martensite. The low working hardening rate of the TSCCR sheet facilitates the forming layered structure of ferrite and martensite during necking compared to the CR sheet. Consequently, the HER of the TSCCR sheet sharply increased by 144% compared to the CR sheet. This enhancement in HER can be attributed to the crack initiation area, longer crack propagation path, and higher crack propagation resistance facilitated by the ductile martensite, culminating in superior HER.

Exploration of the causes of abnormal mold level fluctuation in thin slab continuous casting mold

Abnormal fluctuations in the mold can easily lead to slag entrainment during the thin slab continuous casting process. It is difficult to guarantee the uniform growth of the shell solidification of the slab. The quality of the slab is seriously affected. In this study, the data on thin slab mold level fluctuation were collected from a complete casting sequence of low-carbon steel in a Chinese plant. The discrete wavelet transform (DWT) method is used to decompose the mold level fluctuation and different process parameters. The frequency composition of abnormal fluctuations was clarified, and the causes of the abnormal fluctuations were revealed. The results show that the frequency ranges of abnormal fluctuations are concentrated in 0.3125-0.625 Hz and 0.625-1.25 Hz, and the low set mold level is where the maximum amplitude occurs. Within the time-frequency range, changes in parameters such as electromagnetic braking, mold oscillation, casting speed, stopper rod, and tundish weight cannot be completely matched to abnormal fluctuations collected by the eddy current sensor (ECS). It was found that the abnormal fluctuations in the ECS were that its equipment was disturbed through the analysis of the fluctuations collected by the radioactive source detection (RSD). The degree of disturbance is related to the height of the set mold level. The maximum fluctuation difference exceeded 6 mm between different collection methods when the set mold level was low. In this plant, the position of the ECS is to avoid the area of violent fluctuations of the surface molten steel and be moved appropriately towards the submerged entry nozzle (SEN) to reduce disturbance. When the current ECS position is unchanged, the mold level is set at approximately 305-309 mm, and the casting speed is matched to 4.8 m/min, which minimizes the disturbance with the ECS measurement.

Study of Li2O reduction in mould slag for continuous casting of low-carbon steel slab

The addition of Li2O increases production costs for continuous casting, despite its enhancement of the melting and flow properties of mould slag. Initially, laboratory research focused on assessing the individual impacts of Li2O, F, Na2O and Al2O3 on the viscosity and melting temperature of the slag, using a basicity of 0.95. Subsequently, four stages of industrial testing were conducted to optimise the slag, evaluating parameters such as temperature and friction force curves of the mould copper plate thermocouple, steel slag consumption per ton and strand surface quality. The optimised slag achieved a basicity of 0.98, a viscosity of 0.29 Pa·s and a melting temperature of 1115 °C, meeting all performance requirements for low-carbon steel mould slag. Compared to the original flux, the mass percentage of Li2O decreased significantly from 1.54 to 0 wt-%, while Na2O increased from 2.36 to 7.35 wt-%, and Al2O3 decreased from 7.28 to 3.49 wt-%. The operation of the optimised flux remained stable, with smooth fluctuations observed in the mould copper plate thermocouple temperature and friction force curves. These research findings hold significant reference and practical value for further development and application.

Numerical simulation of non-uniform solidified structure growth of continuous casting slab based on cellular automaton finite-element model

Numerical simulation of non-uniform solidified structure growth of continuous casting slab based on cellular automaton finite-element model

Analysis of Flow Pattern and Bubble Behavior in Slab Continuous Casting with Mold Electromagnetic Stirring

The electromagnetic stirring (EMS) and argon blowing are the widely recognized and extensively employed technologies to control the flow pattern in slab continuous casting, and their interaction is directly linked to the final product. To investigate the flow pattern, bubble characteristics under mold EMS and argon blowing, a mathematical model coupling the electromagnetic field, two‐phase flow is constructed. The population balance model (PBM) under the Eulerian frame is applied to describe the bubbles breakage and coalescence. The results show that the flow regime under the interaction of argon blowing and EMS transfers from bubble ascending flow to intermediate flow to horizontal recirculation flow as EMS current increases, and the large argon flow rate could suppress the formation of horizontal recirculation flow. Meanwhile, EMS can promote the coalescence of bubbles, with the increasing EMS current, the bubble numbers decrease first and then increase, and the bubble mean diameter increases first and then decreases. For obtaining the horizontal recirculation flow, the suitable argon flow rate and EMS current should be matched suitably.

Study on the inverse problem of thin slab continuous casting mold based on fluid-solid coupled heat transfer

The thin slab continuous casting (TSCC) process employs a funnel-shaped mold, introducing increased complexity to both the structure and heat transfer of the mold, as well as the cooling water channels. This paper presents a three-dimensional fluid-solid coupled model for calculating the heat flux in a thin slab mold. The model is calculated and validated based on temperature data collected from molds and cooling water in the plant. The results show that the deviations of the wide face average heat flux and cooling water temperature difference from the measured values are 3.49 % and 1.1 %, respectively, demonstrating good agreement. Moreover, the influence of the thin slab mold geometry on heat transfer is explored. The heat flux values calculated in this model are approximately 13 % lower than those in the simplified model in the upper part of the original funnel area, and 2–7% lower in the lower part. Therefore, this model reflects the heat flux's three-dimensional characteristics of the funnel-shaped mold, offering a novel approach to calculating the heat flux of the thin slab mold.

Monitoring the thermal state of an ingot with beveled and rounded corners in a slab crystallizer of a CCM

The organization of ingot cooling in the continuous caster mold affects the reliability, productivity and quality of the resulting ingots. Insufficient heat exchange between the solidifying metal and the wall of the mold leads to a decrease in the thickness of the solid shell of the ingot, which causes the formation of surface defects, and when the ingot is pulled out of the mold, emergency breakthroughs occur. Excessive heat transfer leads to an increase in the thickness of the solid shell, which entails excessive shrinkage of the ingot surface with the formation of a gap between the solid shell of the ingot and the walls of the mold. The appearance of an air gap leads to increased wear of the copper walls of the mold. There is a design of copper walls that makes it possible to reduce the normal component of forces that occurs during wear, reduce the temperature gradient and stress concentration in the corner of the ingot, and also bring the thickness of the hard crust stocking in the corner closer to the perimeter of the mold. However, the thermal state of the formed ingot with such a crystallizer design remains poorly understood. The goal of the work is to create a virtual tool for monitoring the operation of the mold of a slab continuous caster based on the development of a mathematical and computer model of the thermal state of an ingot with beveled and rounded corners. The temperature distribution in the ingot was calculated using the quasi-equilibrium theory of solidification. The numerical implementation of the mathematical model and obtaining an approximate solution using a computer were carried out using the finite dif-ference method and an implicit difference scheme. A computer program was developed in the Matlab development environment. Using computer modeling, the temperature distribution over the volume of the slab at the outlet of the mold with beveled and rounded corners was obtained. The program for the selected mold design allows you to analyze the solidification of the ingot skin and monitor this process for the selected steel grade and specified casting technological parameters

Large eddy simulation of various EMBr effects on the fluid flow, heat transfer and solidification process in an ultra-high speed thin slab casting mould with multi-port SEN

Effective control of intense turbulence is a crucial challenge for achieving steady production with ultra-high casting speed in thin slab continuous casting. In thin slab casting process, specially designed multi-port submerged entry nozzle (SEN) with four outlets is utilised to ensure an ample supply of molten steel, necessitating the selection and optimisation of suitable Electromagnetic Braking (EMBr) equipment for steel jet control. This study established a comprehensive three-dimensional model of a funnel-type mould, employing a combined experimental-numerical approach to validate and investigate the flow, heat transfer, solidification and electromagnetic behaviour in the mould. The steel grade studied in the simulation is Q235B, and its physical properties were calculated based on its composition with a temperature range of 1450–1826 K. To analyse the influence of high casting speed on the flow and solidification behaviour in the mould, three casting speeds were selected for the study, which were 6, 7 and 8 m/min. The results indicate that the novel Bowl EMBr significantly suppressed the penetration of steel jet and thus enhanced the thickness and uniformity of the solidified shell. As the casting speed increases from 6 to 8 m/min, the solidified shell thickness at the mould exit decreases from 7.91 to 5.94 mm, with the stagnant growth region approaching the mould exit. This highlights the requirement to correspondingly increase EMBr strength under ultra-high casting speed condition to avoid remelting of the shell and the risk of molten steel leakage. The coupled mathematical model established in this study provides guidance for optimising EMBr structures and casting speed under special multi-port SEN conditions, offering recommendations for the rational control of flow, heat transfer and solidification process in the mould.

Influence of Different Submerged Entry Nozzles for Continuous Casting of Ultrathick Slab

Controlling the flow activity at the meniscus in the mold during the continuous casting of ultrathick slab is challenging due to the complex flow behavior, which is not extensively documented. Herein, a multiphase transient model of the ultrathick slab mold has been developed to describe the flow of molten steel, velocity distribution, and stream pattern at different submerged entry nozzle (SEN) outlets. The position of impact points and the fluctuation of liquid level are discussed as well. After ejecting from the port, the stream of the S‐shaped SEN splits into upper and lower streams. The upper stream moves to the liquid level, which improves the liquid level activity near the SEN. However, the reduced mainstream momentum alleviates the occurrence of slag layer exposure. Industrial application of the S‐shaped SEN results in uniform slag distribution and smooth casting operations.

Effect of charging temperature of continuous casting slab on the surface cracks of HSLA steel thick plate

In order to investigate the formation mechanism of the surface cracks on high-strength low-alloy (HSLA) steel thick plates, this paper simulated the temperature history of different hot charging processes, and deduced the microstructural transformation behavior and recrystallization austenitization process under different charging temperatures. This paper proposed four hot charging modes and ultimately elucidated the effect of charging temperature on the surface cracks of HSLA steel thick plates. Experimental results showed significant differences in the microstructure and recrystallized austenite grains under different charging temperatures. Analysis revealed that the recrystallized austenite grains were determined by the microstructure. The presence of austenite in the microstructure significantly increased the size of recrystallized austenite grains, thereby disrupting the uniformity of grains. Based on the relationship between charging temperature, microstructure, and recrystallized austenite grain state, the hot charging process of HSLA steel was divided into four temperature zones: high-temperature single-phase zone, high-temperature two-phase zone, medium-temperature three-phase zone, and low-temperature two-phase stable zone. The occurrence of the surface cracks in HSLA steel was caused by the recrystallized austenite mixcrystal structure generated during charging in the medium-temperature three-phase zone. The uneven recrystallized austenite grains caused by undercooled austenite were the direct cause of the surface cracks, while high charging temperature was the root cause. When the charging temperature entered the medium-temperature three-phase temperature zone, recrystallized austenite grains would exhibit mixcrystal structure. These mixcrystal structure deteriorated the surface quality, promoting the formation, deformation, and propagation of surface cracks. To reduce the occurrence rate of surface cracks, for steel grades sensitive to surface quality, the charging temperature should be controlled in the low-temperature two-phase stable zone.

Effect of the asymmetric flow behaviour of molten steel on solidified shell in slab continuous casting mould

The flow behaviour of molten steel in the mould has an important influence on the solidification quality of the slab. A three-dimensional transient mould model has been developed to describe the effect of the asymmetric flow behaviour of molten steel on solidified shells in the mould. The causes of the asymmetric behaviour of molten steel in the mould were discussed, and the interaction between deviation flow and solidified shell was analysed. The results show that the asymmetric distribution of the flow field is caused by the difference in flow rate at the outlets on both sides of the nozzle and the irregular interaction between the molten steel and the complex nozzle geometry. The level fluctuation is correlated with the deviation flow of the molten steel on the opposite side. The velocity variation trend of the two models is similar, but the fluctuation amplitude of the velocity on both sides of the nozzle in the non-solidification model is more drastic. The conversion period of the deviation flow in the solidification model is longer than that in the non-solidification model. Moreover, the velocity and angle of incident and return flow on both sides are different due to the deviation flow, which mainly affects the solidification of the shell at and below the impact point of the molten steel. The deviation flow will strengthen the impact of the main stream on the shell, resulting in a significant decrease in the thickness at the impact point, while the thickness of the shell below the impact point fluctuates greatly.

Effect of cleaning parameters on the molten pool shape and molten slag dynamics in the corner scarfing process of the casting slab

In the steel industry, improving the hot charging and heat delivery ratio in the continuous casting of slabs and thick slabs can effectively reduce carbon emissions, lower energy consumption, and improve production efficiency. However, this puts very strict requirements on the surface quality of continuous-casting slab. Flame scarfing (or cleaning) technology is currently an important process in the metallurgy field, which can effectively remove impurities and crack defects on the surface and corners of continuous casting slab to ensure the quality and performance of the final product. Under the action of cleaning flame and iron oxide reaction heat, a cleaning molten pool is formed in the corner of the casting slab, and the shape and depth of the molten pool have an important influence on the cleaning effect. A three-dimensional mathematical model is developed to study the effect of slab cleaning speed and oxygen flow rate on the shape of the molten pool in the corners for the corner flame cleaning process. Simultaneously, a two-phase flow volume of fluid (VOF) model for gas-slag interactions is employed to examine the motion behavior of the molten slag. The results showed that when the operating conditions changed, the cleaning speed increased by 1 m/min, the length of the wide and narrow sides decreased by approximately 9.2 mm, and the depth of the corner pool decreased by approximately 5.3 mm. Moreover, a slag-blocking device was designed to mitigate the issue of slag splashing effectively based on the numerical simulation results compared with and without the slag-blocking device. These study works provide beneficial theoretical support and practical guidance for refining and optimizing flame-cleaning techniques.

Numerical study on the effect of high efficient cooling nozzles and varying cooling intensity on metallurgical transport behaviors during the slab continuous casting

In this study, the spray characteristics of the cooling water flux of the traditional single nozzle and novel dual nozzle were innovatively and effectively incorporated into a 3D/2D flow-temperature-concentration segmented model. The model was used to investigate the effects of spray characteristics on the flow, heat distribution, solute transport, solidified shell, and mushy zone of steel in the continuous casting. The results showed that various flow patterns of liquid steel in the turbulent zone significantly affected the temperature and carbon concentration distribution. Until Zone 7, the cooling water fluxes in the six cases remained unchanged. The peak temperatures of cases 1 and 4 in Zone 7 were 1253.11 and 1273.51 K, respectively, indicating that the spray characteristic was the primary cause of the variations in slab surface temperatures. The cooling water fluxes in the six cases change from Zone 8 onward. The six cases had differences of −21.39, 17.87, 46.95, −22.08, 16.86, and 45.68 K between the initial and final temperatures in Zone 8, respectively, meeting the requirement of keeping the maximum temperature recovery rate of slab surface under 100 °C/m. However, the final temperatures for cases 2, 3, and 6 were 1225.62, 1196.50, and 1218.91 K, respectively. These temperatures fall within a realistic plastic temperature range, which must be higher than 1220 K. As a result, the plastic cracking in the slab was possible. The maximum temperature gradient differences of the six cases between feature lines at the end of Zone 8 were 0.793, 0.814, 0.829, 0.185, 0.179, and 0.179 K/mm. These results showed that the optimization effect on the temperature gradient difference was insignificant as the cooling intensity rose. However, the carbon concentration uniformity was marginally improved by increasing the cooling water flux. Finally, a state-owned steel company in China chose case 5 (dual nozzle with moderate cooling intensity at the slab solidified end) as the optimum option for the slab continuous casting caster. The dual nozzle enhanced and promoted the efficient and homogeneous production of the metallurgical process.

Mathematical Modeling of Transient Submerged Entry Nozzle Clogging and Its Effect on Flow Field, Bubble Distribution and Interface Fluctuation in Slab Continuous Casting Mold

Submerged entry nozzle (SEN) clogging will affect the production efficiency and product quality in the continuous casting process. In this work, the transient SEN clogging model is developed by coupling the porous media model defined by the user-defined function (UDF) and the discrete phase model (DPM). The effects of the transient SEN clogging process on the flow field, the distribution of argon gas bubbles and the fluctuation of the interface between steel and slag in the concave bottom SEN in the continuous casting slab mold with a cross-section of 1500 mm × 230 mm are studied by coupling transient SEN clogging model, DPM and volume of fluid (VOF) model. The results show that the actual morphology and thicknesses of SEN clogging are in good agreement with the numerical simulation results. The measurement result of the surface velocity is consistent with the numerical simulation result. With increasing the simulation time, the degree of SEN clogging increases. The flow velocities of molten steel flowing from the outlet of the side hole increase, because the flow space is occupied with the clogging inclusions, which leads to the increased number of argon gas bubbles near the narrow wall. The steel–slag interface fluctuation near the narrow walls also increases, resulting in the increased risk of slag entrapment.

Fluid Flow, Solidification and Solute Transport in Slab Continuous Casting with Different S-EMS Installation Positions

During continuous slab casting, strand electromagnetic stirring (S-EMS) has a significant effect on improving the slab quality. In the current work, a numerical model based on the practical slab continuous casting machine and coupled electromagnetic field, flow field, solidification, and solute transport was established to investigate and evaluate the effect of the S-EMS installation position with various current intensities on metallurgical behavior. The model was verified by magnetic field measurement, infrared camera, and nail shooting experiments. The results show that moving the S-EMS installation position to the solidification end reduces the stirring effect due to the skin effect and the increasing thickness of the slab shell. A higher installation position is beneficial for improving the equiaxed grain rate, while a lower one is beneficial for reducing carbon segregation. The maximum segregation index and range decrease from 1.26 to 1.2 and from 0.42 to 0.36 with the installation position being decreased from −3 m to −12.8 m, respectively. The industrial trials show that S-EMS installed at 3 m has a significant effect on expanding the equiaxed grain zone and a deteriorating effect on reducing carbon segregation.

Numerical Simulation of Segregation in Slabs under Different Secondary Cooling Electromagnetic Stirring Modes.

Secondary cooling electromagnetic stirring (S-EMS) significantly impacts the internal quality of continuous casting slabs. In order to investigate the effects of S-EMS modes on segregation in slabs, a three-dimensional numerical model of the full-scale flow field, solidification, and mass transfer was established. A comparative analysis was conducted between continuous electromagnetic stirring and alternate stirring modes regarding their impacts on steel flow, solidification, and carbon segregation. The results indicated that adopting the alternate stirring mode was more advantageous for achieving uniform flow fields and reducing the disparity in solidification endpoints, thus mitigating carbon segregation. Specifically, the central carbon segregation index under continuous stirring at 320 A was 1.236, with an average of 1.247, while under alternate stirring, the central carbon segregation index decreased to 1.222 with an average of 1.227.

Formation cause and oxidation behaviour of surface crack on the hot-rolled thick plate of high strength low alloy steel

The formation causes and oxidation behaviour of two types of small-sized and irregular morphological surface cracks on S355 hot-rolled thick plate were investigated. The study employed surface pickling, microstructure analysis and inclusion observation to characterise the cracks. It was observed that the surface morphology of two types of cracks exhibited similarities, while their subsurface morphology differed. The Type-A cracks displayed a bulge shape with a rounded tail, while the Type-B cracks had a V-shaped morphology with a slender tail. The Type-A cracks predominantly consisted of FeO. The surface part of the Type-B cracks contained unusual large-sized carbon particles and Al2O3 inclusions, and the iron oxides within the subsurface part followed the sequence of FeO–Fe2O3–Fe from the centre of the cracks to the steel matrix. It was observed that the Type-A cracks originated from subsurface bubbles in the continuous casting slab, which deformed and developed into surface cracks during the rolling process. The iron oxide inside these cracks was formed through a single oxidation process, and the bulge shape in the subsurface part was a distinguishing characteristic. The Type-B cracks were attributed to surface microcracks on the continuous casting slab, which were linked to the presence of unusual large-sized inclusions. The iron oxides within these cracks underwent two oxidation processes. The subsurface morphology and oxide characteristics are the significant features to identify these small-sized and irregular morphological cracks.

Comparison of contamination by non-metallic inclusions in continuous cast slabs of special low carbon steel

The paper studies and compares the macrostructure quality of continuously cast slabs of extra low carbon steel grades DX54D, DX56D, DX57D and 006/IF during casting on a curved-type CCM with and without a vertical section. For this purpose the array of production data of casting of steel of the above mentioned grades, totaling 383 melts, was studied. At the same time, 149 melts were cast on four CCMs (conventionally named A, B, C, D) of curvilinear type with a base radius of 8 m, and 234 melts – on a machine of curvilinear type (E) with a base radius of 11 m and a vertical section of 2.7 m length. Contamination of the cast metal with non-metallic inclusions was assessed metallographically by the degree of development of the defect “point heterogeneity”. The presence of a vertical section allows to reduce the degree of development of point heterogeneity from 1.29 to 0.75 points – by 0.54 points or by 41.9% (relative). With a probability of 99.9% we can assert that the difference between the average values of point heterogeneity is not random and is limited by the presence of the vertical section of the CCM