共查询到20条相似文献,搜索用时 46 毫秒
1.
The formation of oscillation marks on the surface of continuously cast slabs has been studied by metallographically examining
slab samples and by performing a set of mathematical analyses of heat flow, lubrication, and meniscus shape in the meniscus
region of the mold. The metallographic study has revealed that, in agreement with previous work, the oscillation marks can
be classified principally according to the presence or absence of a small “hook” in the subsurface structure at the base of
individual oscillation marks. The depth of the oscillation marks exhibiting subsurface hooks varies with the carbon content,
reaching a maximum at about 0.1 pct carbon, while the oscillation marks without hooks show no carbon dependence. The analysis
of heat flow at the meniscus, which is based on a measured mold heat-flux distribution, indicates that depending on the level
of superheat, the meniscus may partially freeze within the period of a typical mold oscillation cycle. Lubrication theory
has shown that, owing to the geometry of the mold flux channel between the solidifying shell at the meniscus and the straight
mold wall, significant pressure gradients capable of deforming the meniscus can be generated in the flux by the reciprocating
motion of the mold relative to the shell. A force balance on the interface between the steel and the mold flux has been applied
to compute the shape of the meniscus as a function of the pressure developed in the lubricating flux at different stages in
the mold oscillation cycle. This has demonstrated that the “contact” point between the meniscus and mold moves out of phase
with (by π/2), and has a greater amplitude than, the mold displacement so that just at, or near, the end of the negative strip
time molten steel can overflow at the meniscus. From these studies a reasonable mechanism of oscillation-mark formation emerges
which involves interaction between the oscillating mold and the meniscusvia pressure gradients in the mold flux, meniscus solidification, and overflow. The mechanism is consistent with industrial observations.
E. TAKEUCHI, on study leave from Nippon Steel Corporation 相似文献
2.
A. Badri T. T. Natarajan C. C. Snyder K. D. Powers F. J. Mannion A. W. Cramb 《Metallurgical and Materials Transactions B》2005,36(3):355-371
Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel,
originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a
continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop
an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed
to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain
solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations.
The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating
that it is possible to simulate the continuous casting process by a laboratory scale simulator. 相似文献
3.
Improvement of surface defects in a continuously cast slab or billet is important for saving surface scarfing and for carrying out direct rolling. Deep oscillation marks, one type of the surface defects, sometimes cause surface cracking and positive segregation. In this study, the mechanism by which oscillation marks are formed was investigated by using a continuously cast simulator, which is a billet‐type machine. Then attempts were made to reduce the depth of oscillation marks by two methods in which electromagnetic force, which is the most effective means for reducing depth, was not used. The two methods were use of an adiabatic board for preventing solidification of a meniscus and the use of a board for suppressing the flow of flux. Overlapping of a molten metal on a meniscus resulted in formation of oscillation marks in tin. On the other hand, bending of a solidified shell also resulted in formation of oscillation marks in a tin‐lead alloy. The depth of the oscillation marks formed by the overlapping mechanism was greater than that formed by the bending mechanism. Both mechanisms depended on the strength of the solidified shell. Therefore, two trials to reduce the depth of oscillation marks formed by the overlapping mechanism were carried out. In one trial, an adiabatic board was inserted into the molten metal. Reduction in depth of the oscillation marks reduced up to about 86% was achieved when high viscosity flux was used. However, the adiabatic board was not effective when low viscosity flux was used. In the other trial, a board was inserted into a molten flux layer with a depth of 10 mm in depth in order to suppress the flow of the flux and to change the direction of flux flow, and the depth of oscillation marks was reduced by about 33%. Therefore, both of these methods are effective for reducing the depth of oscillation marks in a continuously cast billet. 相似文献
4.
《钢铁研究学报(英文版)》2015,(12)
The meniscus shell plays an important role in slab quality and process operation for continuously cast steel.One decisive reason is initial solidifying shell and growing dendrite under the mechanical stress caused by mold oscillation and liquid steel flow to generate disturbance of casting.The mechanical state of meniscus shell was analyzed using mathematical models in combination with thermo-physical properties and flow rate of steel to shed light on the formation of initial defects.The results show that the mold oscillation is a critical factor on the initial crack formation because the periodic stress makes the shell bending.The formed crack may also expand and propagate due to the following secondary cooling and straightening behavior.The primary dendrite has high possibility to be broken by fluid flow in the solidification front to lead to the non-uniform thickness of solidifying shell.The inter-dendrite bridging is also likely to be formed to produce other internal defects,such as air hole and solute enrichment in the residual molten steel located in the bridging area. 相似文献
5.
复合电磁场对连铸结晶器弯月面金属液运动
行为及铸坯质量的影响 总被引:6,自引:1,他引:5
为了获得高质量的连铸坯 ,提出了在连铸结晶器外施加复合电磁场的电磁铸造方法。规范和测定了冷坩埚式铜铸型内复合电磁场的分布 ;采用低熔点金属镓和 Sn- 4.5 % Pb合金模拟高熔点钢 ,研究了在水冷铜铸型外施加复合电磁场对结晶器弯月面处金属液运动行为及铸坯质量的影响。实验结果表明 :复合电磁场能够有效地抑制结晶器弯月面处金属液波动变形 ,并改善铸坯的表面质量。随着搅拌线圈磁通密度的增加 ,铸坯的凝固组织由柱状晶转变为等轴晶 ,使晶粒得到细化 相似文献
6.
Jung-Wook Cho Shin Yoo Min-Seok Park Joong-Kil Park Ki-Hyeon Moon 《Metallurgical and Materials Transactions B》2017,48(1):187-196
An innovative continuous casting process named POCAST (POSCO’s advanced CASting Technology) was developed based on molten mold flux feeding technology to improve both the productivity and the surface quality of cast slabs. In this process, molten mold flux is fed into the casting mold to enhance the thermal insulation of the meniscus and, hence, the lubrication between the solidifying steel shell and the copper mold. Enhancement of both the castability and the surface quality of high-aluminum advanced high-strength steel (AHSS) slabs is one of the most important advantages when the new process has been applied into the commercial continuous casting process. A trial cast of TWIP steel has been carried out using a 10-ton scale pilot caster and 100-ton scale and 250-ton scale commercial casters. The amount of mold flux consumption was more than 0.2 kg/m2 in the new process, which is much larger than that in the conventional powder casting. Trial TWIP castings at both the pilot and the plant caster showed stable mold performances such as mold heat transfer. Also, cast slabs showed periodic/sound oscillation marks and little defects. The successful casting of TWIP steel has been attributed to the following characteristics of POCAST: dilution of the reactant by increasing the slag pool depth, enlargement of channel for slag film infiltration at meniscus by elimination of the slag bear, and decrease of apparent viscosity of the mold slag at meniscus by increasing the slag temperature. 相似文献
7.
Dong Zhou Wanlin Wang Haihui Zhang Fanjun Ma Ken Chen Lejun Zhou 《Metallurgical and Materials Transactions B》2014,45(3):1048-1056
The initial shell solidification of liquid steel in the mold has significant influence on both surface and internal quality of the final slab, and it is mainly determined by the high transient high temperature thermodynamics occurring in the mold. This study investigated the effects of casting parameters like casting temperature, mold oscillation frequency, and stroke on the initial solidification of a Sn-Pb alloy through the use of a mold simulator to allow the clear understanding of the inter-relationship between irregular shell solidification, heat transfer, negative strip time (NST), and casting conditions. Results suggested that the shell surface oscillation marks (OMs) are strongly depending upon the fluctuations of meniscus responding temperatures and heat flux. An abrupt sudden fluctuation of high frequency temperature and heat flux at the meniscus during the NST would deteriorate the shell surface and leads to deep OMs. The fluctuations of responding temperature and heat flux are determined by the NST, meniscus solidification, and oil infiltration, which in turn are influenced by casting conditions, like casting temperature, oscillation frequency, stroke, etc. 相似文献
8.
The meniscus free casting technology was jointly developed by CRM (Centre de Recherches Métallurgiques) and Irsid (Central Research Laboratory of Usinor) in order to significantly improve the surface and subsurface quality of continuously cast long products. This technology realizes a shifting of the meniscus zone upstream of the solidification area by means of a refractory feed-head located above the mould. In this way, the hydrodynamical perturbations of the meniscus and the meniscus shape do not affect anymore the shell solidification. An important part of the development work was dedicated to the understanding of the solidification mechanisms at the very top of the mould, where the first solidified shell is formed. For that purpose, EMN (Ecole des Mines de Nancy) and Irsid have developed a physical and numerical model of the shell solidification in the new concept. This model takes into account the relative motion between the mould and the product together with two stages of solidification: static near the upper edge of the copper mould and dynamic at the contact of the skin of the cast product. During the casting trials realized at CRM and Irsid, the temperature of the copper on the first 50 mm of the mould and the heat flux extracted were recorded. These values were used to determine the boundary conditions entered into the model. The model calculates the location of the isotherms and the iso-fraction of solid along the mould wall at each moment of the oscillation cycle. It can predict whether or not the skin is sufficiently solid at the end of the negative strip to be extracted without any damage. When the casting speed increases, the resistance of the skin decreases. In order to alleviate this effect, the influence of the oscillation parameters was studied during continuous casting trials. Sinusoidal and non-sinusoidal oscillation cycles were investigated. A triangular cycle brings good surface quality for the high carbon grades while a sinusoidal cycle is better suited to the casting of the peritectic and low carbon steel grades. These results are discussed and a guide for the choice of the best oscillation parameters is provided. 相似文献
9.
钢的软接触电磁连铸技术的研究进展 总被引:1,自引:0,他引:1
钢的软接触电磁连铸技术(SoftContactElectromagneticContinuousCasting)是利用高频交变电磁场在结晶器内铸坯初始凝固区施加电磁压力来减少钢液与结晶器壁的接触压力,从而减小结晶器振动对铸坯表面质量的影响,降低拉坯阻力和减弱初始凝固点的传热来提高铸坯表面质量。分析了实现钢的软接触电磁连铸在结晶器结构、材质以及电磁场参数等方面需要解决的问题,并介绍了该技术的最新研究成果:高频调幅磁场及无结晶器振动的电磁连铸技术。 相似文献
10.
A. Badri T. T. Natarajan C. C. Snyder K. D. Powers F. J. Mannion M. Byrne A. W. Cramb 《Metallurgical and Materials Transactions B》2005,36(3):373-383
The restrictions on quality for low carbon continuously cast slab products require that surface defects be kept to a minimum.
Currently, the steel industry has developed a wealth of experience on how to apply slabs with oscillation marks to very demanding
applications. However, these practices circumvent the problem, rather than solving it. By understanding the formation mechanism
of oscillation marks, one can then develop casting practices that can minimize their effect on slab surface quality. The techniques
developed in this study allowed a more detailed examination of the mold heat-transfer interactions during continuous casting,
such that the variations of heat flux due to irregular solidification near the meniscus could be measured. It is shown that
the mechanisms proposed in the literature are not individually sufficient for the formation of an oscillation mark, but that
several are necessary and must occur in sequence for an oscillation mark to form. A mechanism is proposed for the formation
of oscillation marks that is shown to be in agreement with the trends observed and reported in the literature. Additionally,
it is shown that the success of practices used in industry to reduce the severity of oscillation marks can be explained using
this proposed hypothesis. 相似文献
11.
摘要:为了研究高频磁场下连铸保护渣在结晶器内的润滑状况,建立了高频磁场下连铸保护渣润滑行为数学模型,并应用该数学模型研究了初始凝固时磁场作用下渣道宽度、弯月面高度、渣道动压、渣耗、摩擦力等因素对保护渣润滑行为的影响。结果发现,磁场的作用拓宽了保护渣渣道宽度,增大了弯月面高度,使保护渣渣道入口及出口宽度增加,使初始凝固点下移,改善了传热条件有利于铸坯表面质量提升;磁场的作用减小了因结晶器振动而产生的正压和负压,并且正、负压都是随着磁场强度的增大而减小,但磁场强度存在一个最佳值;磁场的作用增大了渣耗量,改善了铸坯与结晶器之间的润滑;铸坯与结晶器之间摩擦力随着磁场强度的增大而减小,当磁场强度为40mT时,总摩擦力减小趋于平缓,因此磁场强度为40mT左右时对减小摩擦力的作用效果较好。 相似文献
12.
Haihui Zhang Wanlin Wang Dong Zhou Fanjun Ma Boxun Lu Lejun Zhou 《Metallurgical and Materials Transactions B》2014,45(3):1038-1047
The importance of initial solidification of molten steel in the mold has been widely acknowledged. However, very few studies have been effectively developed because of the high transient nature of thermodynamics and fluid flow in the upper mold. Based on the recently developed mold simulator technology, a novel technique has been successfully developed to study the initial solidification behavior of Sn-Pb alloy, which gives rise to the clear understanding of the interrelationship between complex meniscus heat transfer, casting surface oscillation marks (OM), and mold hot-surface responding temperatures. The results suggested that the variations of the responding temperatures and heat flux at meniscus may be associated with the movement of mold in/out of the bath, the infiltration of silicon oil, and the latent heat release due to the solidification of meniscus during negative strip time (NST). The presence of positive peaks in the derivative of the heat flux are corresponding to each of the OM during NST, which suggests the significant increase of heat flux during the formation of OM. These could be explained as the meniscus is deformed and gets closer to the coldest mold at the beginning of NST, such that the liquid meniscus that gives rise to the increase of heat flux would be solidified. With the enhancement of oil infiltration from the mid-NST to end-NST, the thermal resistance between the solidified meniscus and mold decreases; therefore, the shell continues to grow, and the resulting heat-transfer and mold temperatures also continue to increase. 相似文献
13.
Pavel E. Ramirez Lopez Kenneth C. Mills Peter D. Lee Bego?a Santillana 《Metallurgical and Materials Transactions B》2012,43(1):109-122
Oscillation marks (OMs) are regular, transverse indentations formed on the surface of continuously cast (CC) steel products.
OMs are widely considered defects because these are associated with segregation and transverse cracking. A variety of mechanisms
for their formation has been proposed (e.g., overflow, folding, and meniscus freezing), whereas different mark types have also been described (e.g., folded, hooks, and depressions). The current work uses numerical modeling to formulate a unified theory for the onset of
OMs. The initial formation mechanism is demonstrated to be caused by fluctuations in the metal and slag flow near the meniscus,
which in turn causes thermal fluctuations and successive thickening and thinning of the shell, matching the thermal fluctuations
observed experimentally in a mold simulator. This multiphysics modeling of the transient shell growth and explicit prediction
of OMs morphology was possible for the first time through a model for heat transfer, fluid flow, and solidification coupled
with mold oscillation, including the slag phase. Strategies for reducing OMs in the industrial practice fit with the proposed
mechanism. Furthermore, the model provides quantitative results regarding the influence of slag infiltration on shell solidification
and OM morphology. Control of the precise moment when infiltration occurs during the cycle could lead to enhanced mold powder
consumption and decreased OM depth, thereby reducing the probability for transverse cracking and related casting problems. 相似文献
14.
In a study of early solidification during the continuous casting of steel slabs, the effect of the formation of oscillation
marks on the surface quality of the slabs has been examined by metallographic in-vestigation of slab samples and by performing
a set of mathematical analyses. Positive segregation of solute elements, especially phosphorus and manganese, has been observed
at the bottom of the oscillation marks and has been classified into two categories. One type is observed at the end of the
overflow region on subsurface hooks which originate from partial solidification of the meniscus. A heat-flow model which takes
into account the shape of the oscillation marks has revealed that this type of positive segregation is caused by local delay
of solidification at the bottom of the oscillation marks. The other type of positive segregation has been found in a layer
on the bottom of the oscillation marks without subsurface hooks. This form of segregation cannot be explained by the heat-flow
model, but is likely due to a penetration mechanism in which the negative pressure in the flux channel generated during the
upward motion of the mold draws out interdendritic liquid from the semi-solidified shell. Transverse cracks are found along
the bottom of oscillation marks. The surface of the transverse cracks exhibits an interdendritic appearance in the vicinity
of the slab surface, which implies that the cracks are initiated as hot tears in the mold region. A heat-flow analysis predicts
that deep oscillation marks cause nonuniformity of the shell in the mold, which also was observed in the metallographic in-vestigation.
According to the heat-flow analysis, not only the depth but also the pitch of the oscillation marks affects the shell profile.
Therefore increasing the frequency of mold oscillation effectively reduces transverse cracks, by decreasing both the depth
and the pitch of oscillation marks.
Formerly Graduate Student in the Department of Metal-lurgical Engineering, University of British Columbia
An erratum to this article is available at . 相似文献
15.
Joydeep Sengupta Brian G. Thomas Ho-Jung Shin Go-Gi Lee Seon-Hyo Kim 《Metallurgical and Materials Transactions A》2006,37(5):1597-1611
The initial stages of solidification near the meniscus during continuous casting of steel slabs involve many complex inter-related
transient phenomena, which cause periodic oscillation marks (OMs), subsurface hooks, and related surface defects. This article
presents a detailed mechanism for the formation of curved hooks and their associated OMs, based on a careful analysis of numerous
specially etched samples from ultra-low-carbon steel slabs combined with previous measurements, observations, and theoretical
modeling results. It is demonstrated that hooks form by solidification and dendritic growth at the liquid meniscus during
the negative strip time. Oscillation marks form when molten steel overflows over the curved hook and solidifies by nucleation
of undercooled liquid. The mechanism has been justified by its explanation of several plant observations, including the variability
of hook and OM characteristics under different casting conditions, and the relationships with mold powder consumption and
negative/positive strip times. 相似文献
16.
An improved mathematical model for electromagnetic casters and testing by a physical model 总被引:8,自引:0,他引:8
The paper describes a physical model aimed at studying two important phenomena in electromagnetic (EM) casting of aluminum:
the support of the molten metal pool and stirring caused by EM forces. The physical model is used both to test an improved
mathematical model for EM casting and to provide insight into the effect of design changes on the two EM phenomena. Examples
of design changes are changes in inductor current and position and screen position. The improved mathematical model, a two-dimensional
(2-D) (axisymmetric) one, constrains the melt surface at the solidification line and neglects (with justification) buoyancy,
surface tension, and the impact of flow on meniscus shape. The physical model was a cylindrical one where the solidified metal
was simulated by a 248-mm-diameter aluminum bronze cylinder and the molten metal by Wood’s alloy. Measurements were made of
electric field, magnetic field, meniscus deformation, and velocities for the two types of caster in commercial use. Generally,
good agreement was obtained between the mathematical model and the experimental measurements. 相似文献
17.
The surface of continuously cast slabs is characterized by the presence of oscillation marks. Direct linkage of the continuous
casting process and hot rolling process requires that cast slabs should be free of surface defects. In the present work, a
mechanical model has been developed for the prediction of the depth of oscillation marks of the depression type. It is based
on the beam bending theory and on viscoplastic material behavior. The downward movement of the strand is taken correctly into
account, which has not been done in previous models. Auxiliary parts of the model are the models for the determination of
the temperatre field and of the fluid flow and pressure in the meniscus region and in the gap between strand and mold. The
deflection of the shell is computed as a function of time and distance from the shell tip. The retained deflection, which
corresponds to the depth of oscillation marks observed on the slab surface, is determined for different values of stroke,
frequency, and casting velocity. The theoretical data are compared with the measured data as available in the literature. 相似文献
18.
Baokuan Li Toshimitsu Okane Takateru Umeda 《Metallurgical and Materials Transactions B》2000,31(6):1491-1503
A mathematical model has been developed to analyze molten metal flow, considering the effects of argon gas injection and static
magnetic-field application in the continuous casting process. The k-ɛ turbulence model is used to calculate the turbulent variables. A homogeneous fluid model with variable density is employed
to tackle the molten metal-argon gas flow. The electromagnetic force is incorporated into the Navier-Stokes equation, and
the effects of boundary conditions of the magnetic field on the velocity distribution near the mold wall are included. A good
agreement between the numerically obtained flow-field results and measurements is obtained. The argon gas injection changes
the molten metal flow pattern, mainly in the upper portion of the mold. By applying the magnetic field, values of the averaged
velocity field in the bulk decrease significantly, and, especially at the top free surface, they become very small, which
can cause meniscus freezing. When magnetic-field application and argon gas injection are used together, the external flow
field out of the gas plume is significantly suppressed; nevertheless, flotation of gas bubbles is still active and is not
affected directly by the magnetic field. Although the penetrating length of the gas plume is shortened, the argon gas bubbles
in molten steel still cause fluctuation at the top free surface, which prevents the occurrence of freezing. 相似文献
19.
基于Fluent建立了150 mm×150 mm结晶器的三维模型,模拟计算了结晶器内流场、温度场及溶质分布的变化,并对二冷区宏观偏析进行了模拟.结果发现,结晶器角部传热方式为二维传热,与表面一维传热相比凝固速度较快.结晶器角部钢液存在回流,同时弯月面处钢液也存在小的回流.受回流及凝固的影响,碳元素在结晶器内会重新分配,上部表现为正偏析、回流通道表现为负偏析.并且发现,由于固液相扩散系数的不同,直到凝固终点,铸坯冷却过程中都会存在环形负偏析. 相似文献
20.
为研究水口结构形状对连铸中低碳钢结晶器内流场和温度场分布的影响,采用有限容积法建立连铸圆坯三维数学模型,模拟了不同水口形状下圆铸坯的流场和温度场。结果表明,在水口浸入深度为80 mm、其他参数不变时,与直水口相比,旋流水口使钢水冲击深度降低,结晶器内涡流增强,弯月面温度和二冷区凝固率提高,且随着水口数量的增加,弯月面波高和结晶器出口温度降低;采用旋流水口并施加结晶器电磁搅拌(M EMS)时,结晶器中钢液温度升高,弯月面有卷渣行为,结晶器出口未形成凝固坯壳。在实际应用中,应避免同时使用M EMS和旋流水口,或使用旋流水口时采用低强度的M EMS。 相似文献