Bloom Surface Research Articles

Effect of nozzle type on fluid flow, solidification, and solute transport in mold with mold electromagnetic stirring

The mathematical model of coupling fluid flow, heat transfer, solidification, solute transport, and the electromagnetic field of the bloom in the upper part of the strand was established with three nozzle types. Then, the flow field, distribution of the temperature, solidification, and macrosegregation of carbon were investigated and compared by numerical modeling. In the case of the straight submerged entry nozzle (SEN), the molten steel flows down deep into the liquid pool, and the depth of the jet flow reaches about 1.0 m beneath the meniscus. The jetting zone is the high-temperature zone. In the case of two-port SEN and four-port SEN, the flow patterns and distribution of temperature in the central longitudinal section are similar. The jet flow impinges directly on the initially solidified shell and then it is divided into two longitudinal circulations. The heat of molten steel is dissipated along with the longitudinal circulations. The negative segregation band was generated near the bloom surface due to the washing effect by the rotating flow at the solidification front with three nozzle types. The negative segregation deteriorates gradually with the number of ports decreasing.

A New Cooling Strategy in Curved Continuous Casting Process of Vanadium Micro-alloyed YQ450NQR1 Steel Bloom Combining Experimental and Modeling Approach

In the continuous casting process of steel, the bloom surfaces would experience intensive cooling from the water-cooled copper mold to secondary cooling water spray. If the cooling process is not controlled properly, hot ductility of the bloom surface microstructures would deteriorate, and bloom surface cracks would form easily under straightening deformation in a curved caster. Considering the above facts, the cooling scheme for the continuous casting of YQ450NQR1 steel bloom, a kind of vanadium-containing micro-alloyed steel, is studied with both experimental investigation and mathematical modeling in this work. The authors first investigate the hot ductility of bloom surface microstructures at various cooling rates using a Gleeble thermal simulator. Then, the precipitation of V(C, N) particles and its influence on ferrite formation during continuous cooling are studied and characterized using High-Temperature Laser Scanning Confocal Microscopy. Based on these, the preferred cooling rate for surface microstructures at the straightening position in the caster is obtained. To further reduce the solute macro-segregation through enlargement of the equiaxed crystal zone, a cellular automaton-finite element model is used to calculate heat transfer and solidification structure evolution during the continuous casting process. After calibration with industrial trials, the model is utilized to determine the critical position for columnar to equiaxed transition and to adjust the pouring temperature for the melt. Combining the above research, a new cooling strategy for YQ450NQR1 steel bloom is obtained, which can improve crack resistance of bloom surface microstructures and reduce solute macro-segregation at the same time.

Model prediction of the effect of in-mold electromagnetic stirring on negative segregation under bloom surface

Aiming at the problem of negative segregation under a bloom surface, a coupling macrosegregation model considering electromagnetic field, flow, heat, and solute transport was established based on the volume average method to study the effect of in-mold electromagnetic stirring (M-EMS) on the negative segregation under the bloom surface. In the model, the influence of dendrite structure on the flow and solute transport was described by the change of permeability. The model was validated by the magnetic induction intensity of M-EMS and carbon segregation experiment. The results show that the solute C in the solidified shell in the turbulent zone of the bloom undergoes two negative segregations, whereby the first is caused by nozzle jet, and the second by the M-EMS. The severities of the negative segregation caused by M-EMS at different currents and frequencies are also different, and the larger the current is, or the smaller the frequency is, the more serious will be the negative segregation. With the M-EMS, the solute C distribution in the liquid phase of the bloom is more uniform, but the mass fraction of C in the liquid phase is higher than that without M-EMS.

Impact of extreme wet and dry years on the persistence of Microcystis harmful algal blooms in San Francisco Estuary

Cyanobacteria harmful algal blooms (CHABs) became a concern in the upper San Francisco Estuary, California beginning in 1999, when yearly blooms of Microcystis began in the Delta region. Subsequent research identified that the increase in the magnitude, duration and toxicity of Microcystis blooms was associated with drought related conditions of elevated water temperature and low streamflow. However, the impact of extreme conditions on the resilience of the bloom was unknown. The 2014 and 2017 water years provided a unique opportunity to determine the effect of climatic “whiplash” produced by the occurrence of extreme wet conditions following extreme dry conditions on the Microcystis bloom. We hypothesized that the period of record wet conditions in 2017 (1906-2018) would eliminate the Microcystis bloom for that year and perhaps revert the estuary phytoplankton community back to pre-bloom conditions due to extreme flushing, despite the increase in magnitude and spatial and temporal distribution of the Microcystis bloom during the 2014 extreme dry year. Field sampling was conducted at 2-week or 4-week intervals between July and November at stations throughout the Delta for both years and included a suite of physical, chemical and biological factors. Using PRIMER-e DISTLM, we determined that retention time in the upper estuary and water temperature were key environmental correlates with the Microcystis bloom amplitude and in regression models described 58-78% of the variation of the bloom surface biovolume or subsurface abundance. The period of record high streamflow in 2017 was not enough to eliminate the Microcystis bloom. However, the bloom was small in 2017, with a low abundance, late initiation, short duration, narrow distribution and low toxin production. Warm water temperature enabled the bloom to flower in late summer despite streamflow many times those measured previously. In addition, although conditions early in the summer of 2017 favored diatoms, the summer was characterized by an abundance of other non-Microcystis cyanobacteria. We conclude that once established, Microcystis is likely to be resistant to extreme wet conditions, as long as water temperature and other key water quality conditions are favorable.

Closure of Internal Porosity in Continuous Casting Bloom During Heavy Reduction Process

To investigate the closure behavior of internal porosity (also referred to as internal void) in continuous casting bloom during heavy reduction (HR) and thus provide theoretical guidance for minimizing this kind of internal defect more effectively with HR, a three-dimensional (3D) mechanical model was developed based on the predicted temperature field by a 2D heat transfer model. With this 3D mechanical model, closure behaviors of internal porosity in continuous casting bloom during HR at and after the strand solidification end under different process conditions were numerically studied. It was found that the void axis length decreased significantly along the bloom thickness direction and increased slightly along the casting and bloom width directions after HR, and the influence of the initial void size on the void closure was not obvious. With a decrease of temperature difference between the bloom surface and center, HR efficiency for minimizing internal void decreased, while the required reduction force significantly increased. Compared with blooms with a uniform temperature distribution of 1100 °C, the void closure index after HR implemented at the strand solidification end was increased by ~ 25 pct. Compared with a conventional flat roll, the application of a convex roll during HR could contribute to minimizing the internal porosity more effectively and significantly enhance the reduction capacity of the withdrawal and straightening units. The void closure index of ηs and ηv (where ηs and ηv were defined based on the variation of the void aspect ratio and the void volume, respectively) was closely related to the equivalent strain (eeq) and the hydrostatic integration parameter (Q), respectively, and two mathematical equations were derived to quantitatively describe the relationship of ηs − eeq and ηv − Q.

Combined Danio rerio embryo morbidity, mortality and photomotor response assay: A tool for developmental risk assessment from chronic cyanoHAB exposure

Freshwater harmful algal blooms produce a broad array of bioactive compounds, with variable polarity. Acute exposure to cyanotoxins can impact the liver, nervous system, gastrointestinal tract, skin, and immune function. Increasing evidence suggests chronic effects from low-level exposures of cyanotoxins and other associated bioactive metabolites of cyanobacterial origin. These sundry compounds persist in drinking and recreational waters and challenge resource managers in detection and removal. A systematic approach to assess the developmental toxicity of cyanobacterial metabolite standards was employed utilizing a robust and high throughput developmental Danio rerio embryo platform that incorporated a neurobehavioral endpoint, photomotor response. Subsequently, we applied the platform to cyanobacterial bloom surface water samples taken from temperate recreational beaches and tropical lake subsistence drinking water sources as a model approach. Dechorionated Danio rerio embryos were statically immersed beginning at four to six hours post fertilization at environmentally relevant concentrations, and then assessed at 24 h and 5 days for morbidity, morphological changes, and photomotor response. At least one assessed endpoint deviated significantly for exposed embryos for 22 out of 25 metabolites examined. Notably, the alkaloid lyngbyatoxin–a resulted in profound, dose-dependent morbidity and mortality beginning at 5 μg/L. In addition, hydrophobic components of extracts from beach monitoring resulted in potent morbidity and mortality despite only trace cyanotoxins detected. The hydrophilic extracts with several order of magnitude higher concentrations of microcystins resulted in no morbidity or mortality. Developmental photomotor response was consistently altered in environmental bloom samples, independent of the presence or concentration of toxins detected in extracts. While limited with respect to more polar compounds, this novel screening approach complements specific fingerprinting of acutely toxic metabolites with robust assessment of developmental toxicity, critical for chronic exposure scenarios.

Effects of different initial pH and irradiance levels on cyanobacterial colonies from Lake Taihu, China

Cyanobacteria usually appear in colonies on the surface of lakes, but the microenvironment inside colonies is not as easily detected. An accurate analysis of microenvironment properties within the colonies is key to a better understanding of the formation mechanism of cyanobacterial blooms. To understand the influence of irradiance and pH on the characteristics of cyanobacterial colonies from Lake Taihu, dissolved oxygen (DO) and pH microelectrodes were used to investigate physiological responses within these colonies and in the motionless water blooms at different irradiances and initial pH levels. The results showed that DO and pH increase with increasing irradiance, causing a dynamic alkaline environment to form inside these colonies. The maximum pH varies from 9 to 9.5 at all initial pH readings and the highest DO was achieved in the colonies incubated at an initial pH of 9. The maximum net photosynthesis (Pn) and dark respiratory rate (Rdark) were achieved in the colonies incubated at an initial pH of 8 and 6, respectively. The maximum pH differences were lower in colonies incubated at an initial pH of 10 compared with those incubated at a pH of 6 to 9. Photosynthesis of the colonies raised the aqueous pH to about 10.5, which is similar to the value found inside the colonies. In the motionless water bloom layer, the maximum pH varies from 10 to 10.5 at all initial pH levels and both the highest DO and pH values were achieved at an initial pH of 10. Cyanobacterial photosynthesis first created an alkaline microenvironment in the colonies and then increased the aqueous pH. This elevated aqueous pH promotes photosynthesis of the colonies and further increases the aqueous pH until it is higher than 10. Abundant oxygen bubbles attached at the colonies surface provide extra buoyancy for the colonies. An anaerobic environment forms at 3 to 4 cm depth under the bloom surface, aggravating the outbreak of cyanobacterial bloom. All these physiological characters of microenvironment in cyanobacterial colonies and water blooms favor cyanobacteria as the dominant water bloom species in eutrophic water.

Numerical Study on Solidification Behavior and Structure of Continuously Cast U71Mn Steel

The solidification behavior and structure of continuous casting high rail U71Mn bloom, under different secondary cooling conditions and superheat, were numerically investigated using the Cellular Automaton-Finite Element (CAFE) model implemented with ProCAST software. Nail shooting and macro etch experiments of the bloom samples under different cooling conditions were conducted to verify the model of macroscopic solidification and structure. The results showed that the simulated results of the solidified shell and solidification structure are basically consistent with experimental results. The secondary cooling condition has little effect on the grain size and distribution of the bloom, while both the bloom surface and corner temperatures are higher and the temperature rise at the beginning of the air-cooling zone is smaller under the super-slow cooling condition. The percentage of center-equiaxed grains decreases from 44.6% to 20.1% and the grain average radius increases from 1.025 to 1.128 mm when the superheat increases from 15 to 40 K, with little change in the grain size occurring between 15 and 20 K. Moreover, for a step increase in the superheat of 5 K, the solidification end is lengthened by about 0.19 m and the surface temperature is enhanced by 3 K. The super-slow secondary cooling condition with the superheat controlled within 20 K is suitable for big-bloom casting.

The Effects of a Submerged Entry Nozzle on Flow and Initial Solidification in a Continuous Casting Bloom Mold with Electromagnetic Stirring

The melt flow, level fluctuation, temperature field, and solidification behavior coupled with electromagnetic stirring (EMS) effects in the continuous casting mold region of U71Mn steel bloom were numerically analyzed by commercial computational fluid dynamics (CFD) software named ANSYS FLUENT. The effects of submerged entry nozzle (SEN) structures and the installation methods for optimized four-port SEN on the flow pattern, level fluctuation, heat transfer and initial solidification behavior in a bloom mold loaded with EMS were investigated. The aim is to propose a better SEN condition for the big bloom casting of high railway steel. The water simulation experiments were conducted to show the flow characteristics under different SEN conditions and verify the numerical model of flow pattern. The experimental and numerical simulation results showed that the optimized four-port SEN with diagonal installation cannot only improve the flow pattern of the molten steel by alleviating the level fluctuation and reducing the impact pressure to the wall. It is also beneficial for temperature variation at both bloom surface and corner, as well as the local solidified shell thinning phenomena due to the elimination of impingement effect.

Effects of EMS Induced Flow on Solidification and Solute Transport in Bloom Mold

The flow, temperature, solidification, and solute concentration field in a continuous casting bloom mold were solved simultaneously by a multiphysics numerical model by considering the effect of in-mold electromagnetic stirring (M-EMS). The mold metallurgical differences between cases with and without EMS are discussed first, and then the solute transport model verified. Moreover, the effects of EMS current intensity on the metallurgical behavior in the bloom mold were also investigated. The simulated solute distributions were basically consistent with the test results. The simulations showed that M-EMS can apparently homogenize the initial solidified shell, liquid steel temperature, and solute element in the EMS effective zone. Meanwhile, the impingement effect of jet flow and molten steel superheat can be reduced, and the degree of negative segregation in the solidified shell at the mold corner alleviated from 0.74 to 0.78. However, the level fluctuation and segregation degree in the shell around the center of the wide and narrow sides were aggravated from 4.5 mm to 6.2 mm and from 0.84 to 0.738, respectively. With the rise of current intensity the bloom surface temperature, level fluctuation, stirring intensity, uniformity of molten steel temperature, and solute distribution also increased, while the growth velocity of the solidifying shell in the EMS effective zone declined and the solute mass fraction at the center of the computational outlet (z = 1.5 m) decreased. M-EMS with a current intensity of 600 A is more suitable for big bloom castings.

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.

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

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

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

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

Influence of Submerged Entry Nozzle on Intermixed Zone in Round Blooms with a Diameter of 525 mm / Wpływ Wylewu Zanurzeniowego Na Strefe Przejsciowa W Okragłych Wlewkach Ciagłych O Srednicy 525 mm

This work compares the experimental results of nickel concentration measurements in the intermixed zone of the continuously cast round blooms with a diameter of 525 mm using two types of submerged entry nozzles (SEN) - a straight-through nozzle and one with 5-ports. Based on determination of the system and optical interface in the blooms a detailed study of concentration profiles on the bloom surface in a small radius area, on the right side and then also on a cross-section of the blooms, was carried out. The results were further analysed using approximation models, and were to be used to verify the proposed model for predicting intermixed zones for a continuous casting machine, developed based on the results of physical and numerical modelling.

The effects of adding water and polyglycerol polyricinoleate on the texture, appearance, and sensory qualities of compound milk chocolate

AbstractWater and polyglycerol polyricinoleate (PGPR) contents were varied to investigate the effects of these parameters on the textural properties, surface color, and sensory qualities of compound chocolates. The content levels of water and PGPR were manipulated between 3–10 and 0.3–3.3%, respectively (content expressed as % by weight of finished product). Simultaneous variations in water and PGPR levels, especially in high ratios, resulted in a drastic reduction in the hardness values (p<0.001), darker color (p<0.01), and an unusual taste (p<0.05) but the effect of water addition was more pronounced than PGPR. It was observed that compound chocolates with 3% water content were not dissimilar from the control with respect to all properties. In the samples of the same water content, the effect of PGPR addition was nearly insignificant. For these confectionaries, the best proportion of ingredients for producing water‐containing compound chocolate was considered the one which has the least negative effects on bloom surface area and the texture.Practical applications: Manufacturing water‐containing imitation chocolates represent a general approach for adding all water‐base materials to chocolate such as cream, yogurt, milk, etc. or water‐soluble substances like trace elements and vitamins. Conventional chocolates become soft at above 28°C, and lose shape retention at above 32°C. Water addition provides a heat‐resistance compound chocolate with shape retention at a temperature above 40°C, being not sticky to the direct touch. However, there has been very limited information about water addition's effects on the chocolate structure. In order to be able to predict the structural variations, it is important to study how water affects the physical properties of the chocolates.

Simulation of thermomechanical behavior during continuous casting process based on MiLE method

A new method called mixed Lagrangian and Eulerian method (MiLE method) was used to simulate the thermomechanical behavior during continuous casting process of steel YF45MnV. The simulation results are basically in agreement with the measured data. The delaying period at the beginning of solidification is about 0.1 in square root of solidification time which is agreement with the data in literatures, and shell thickness increases in linear relation to square root of solidification time. The bloom surface temperature decreases gradually as the casting proceeds. The effective stress in the corner is much larger than that in the mid-face. The corner area is the dangerous zone of cracking. The effects of mold flux break temperature on the air gap and hot tearing indicator were also modeled. The model predicts that the bloom surface temperature increases with the increase of the mold flux break temperature, but the heat flux decreases with the increase of the mold flux break temperature. The hot tearing indicator is much smaller when the mold flux break temperature is higher.

Shape Evolution of Pinholes in Bloom with Multi-Scale Finite Element Technique

This study investigates the shape evolution of the pinholes on the bloom surfaces, which are originated from the continuous casting process, during multi-pass hot bar rolling. It is important to track the shape evolution of the pinholes since they can be formed as the sharp surface cracks after hot bar rolling and can initiate the surface bursts in the cold forging process. It is very hard to track the deformation behavior of the pinholes with detection tools during hot rolling, so the numerical simulations can be properly utilized. In general, the size of the pinholes in the bloom surface is order of micrometer although the bloom size is order of millimeter. This size discrepancy between them makes it difficult to discretize the domain including the pinholes for the finite element (FE) simulations. To overcome this limitation of the conventional FE simulation, multi-scale technique coupling the macro and micro models was developed in current study. This technique was implemented into the commercial simulation code, DEFORM-3D. The developed multi-scale simulation technique was capable of simulating the shape evolution of the pinholes through multi-pass hot bar rolling successfully. It is concluded that aspect ratio of the initial pinhole should be larger than 2.0 approximately to prevent it folded.

Shape Evolution of Pinholes in Bloom with Multi-Scale Finite Element Technique

This study investigates the shape evolution of the pinholes on the bloom surfaces, which are originated from the continuous casting process, during multi-pass hot bar rolling. It is important to track the shape evolution of the pinholes since they can be formed as the sharp surface cracks after hot bar rolling and can initiate the surface bursts in the cold forging process. It is very hard to track the deformation behavior of the pinholes with detection tools during hot rolling, so the numerical simulations can be properly utilized. In general, the size of the pinholes in the bloom surface is order of micrometer although the bloom size is order of millimeter. This size discrepancy between them makes it difficult to discretize the domain including the pinholes for the finite element (FE) simulations. To overcome this limitation of the conventional FE simulation, multi-scale technique coupling the macro and micro models was developed in current study. This technique was implemented into the commercial simulation code, DEFORM-3D. The developed multi-scale simulation technique was capable of simulating the shape evolution of the pinholes through multi-pass hot bar rolling successfully. It is concluded that aspect ratio of the initial pinhole should be larger than 2.0 approximately to prevent it folded.

Causes and Countermeasures of Cracking in Cogging Process of 40Cr Bloom

This paper has analyzed the solidification structure and thermodynamic state of bloom by means of the electron microscopy observation and heat transfer model, and studied the cracking mechanism and influence factors in the cogging process of 40Cr bloom. The results show that the main cracking reasons are the unreasonable secondary cooling scheme, the high reheating temperature in the bloom surface, the coarser grain and widmanstatten structure of the bloom subsurface. When the bloom is conveyed from the steel mill to rolling mill, the bloom may crack for the phase transition stress and thermal stress, some micro-cracks occur in the bloom surface, furthermore, the crack will expand when it is heated in the heating furnace and rolled in the cogging mill, the cracking occurs in bloom eventually. In order to reduce the bloom cracking in the cogging process, it must optimize the secondary cooling system, reduce the cooling intensity, and adopt the thermal insulation, hot delivery and hot charge process. If the bloom can not be conveyed in the hot delivery and hot charge mode, it should be conveyed to the slow cooling pit.

Austenite Grain Refining of As-cast Bloom Surface by Reduction of Oscillation Mark Depth

Austenite grain refining in the surface layer of as-cast bloom is effective for the reduction of surface cracks in steel production. This study was carried out to clarify the influence of cooling rate on the as-cast austenite grain size and its growth mechanism. Depth and interval of oscillation marks were reduced and the austenite grains at the bloom surface were refined under the oscillation conditions of shorter stroke and higher frequency. The average cooling rates were estimated to be from 6 to 16 K/s in austenite phase temperature and the rates were related to characteristics of oscillation mark. It was found that the austenite grain growth direction did not vary even when characteristics of oscillation mark changed. The austenite grain size below the surface layer was also determined by austenite grain size at the bloom surface.