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《钢铁冶炼》2013,40(4):304-309
AbstractMould oscillation is needed to reduce friction and thus prevent sticking and breakout of the liquid metal during casting. However, this oscillation is known to cause surface defects in the solidified steel slabs, so called oscillation marks. In this paper, the depth and the depth variation of these oscillation marks were studied using a two-level full factorial experiment (2) with four additional centre point runs. Four factors were studied: stroke length of the mould, oscillation frequency, motion pattern (strip factor) and casting speed. The stroke length affected the depth of the marks the most, where larger strokes created deeper marks. The interaction between the oscillation frequency and the strip factor of the mould also affected the oscillation mark depth. The oscillation mark depth variation was also increased by increased stroke lengths and at higher oscillation frequencies. The largest effect on the oscillation depth variation was found for the interaction between the stroke length and the oscillation frequency. 相似文献
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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
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By Gleeble- 3500 thermal simulator, the simulation experiments of austenite grain growth and ductility in the third brittle zone on the surface slab were carried out respectively, which identified the growth regularity of austenite size and hot ductility under different cooling rates. The results show that the austenite ??blown grains?? (>1mm) with lower cooling rate (<5??/s) are formed easily. The precipitates of fine Ti(C,N) around the austenite boundary could pin and limit the growth of austenite with the increase of cooling rate. In the thermal stretching experiment, reduction of area of Ti- Nb bearing mirco- alloyed steel slab at 800?? respectively are only 29. 7% and 23. 0% with cooling rate of 1 and 5??/s. Both precipitating rectangle or irregular (Ti,Nb)(C,N) precipitates with 70-200nm size and needlelike Nb(C,N) precipitates with 40-100nm. The formation of ??blown grains?? at the depths of oscillation marks with low cooling rate, and straightening in the third brittle zone are the major causes of high corner transverse crack sensitivity of Ti- Nb micro- alloyed steel slabs. 相似文献
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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
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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
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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
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The micro-characteristics of the streaky marks and the corresponding substrate surface of galvannealed(GA)steel sheets have been investigated using SEM and FIB.The streaky mark area seems to have many cavities, similar to craters,which scatter light and make the surface appear darker when compared with the surrounding normal area.Moreover,it is found that the irregularity or difference in steel grain size tends to affect the surface quality of the GA steel sheets.During alloying,the diffusion speed of Zn-Fe on the grain boundary is faster than that at the center of the grain,which results in the selective formation of crater-like morphology in the coating area with larger steel grains.Due to the clear grain boundary of IF steel,apparent streaky marks will be formed on the coatin,iftheIFsteelsurfaceconsistsofrainsofirreularsize. 相似文献
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In the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%. 相似文献
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Reliable data are limited to the critical strain for the formation of transverse cracks on the slabs, owing to experimental difficulty to simulate temperature gradient in solidified shell in continuous casting mold. The present study is to determine the critical strain, ?c, for the formation of transverse cracks on continuously cast slabs. A convenient and simple hot tensile test using rectangular test pieces with either V-notch or semi-circle notch or oscillation marks has been developed by placing the specimen under similar temperature gradient to that in solidified shell in the mold. The ?c has been determined at a better accuracy and reproducibility, and the ?c at a strain rate of 5?10?4s?1 is found to be a high 35% for test pieces without notch. It sharply decreases, however, to 10% for those with V- and semi-circle-notches, slightly decreases with increasing notch depth, and further decreases for those with oscillation marks that accompany solute segregation. Reduction of the oscillation mark depth is shown to be important measure to prevent the occurrence of transverse cracking of continuously cast slabs. 相似文献
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为控制中厚板中间坯长时间待温导致的晶粒长大,研究了中间强制水冷却对奥氏体组织的影响.通过对Q345B钢和含Nb-Ti钢采用1050℃变形后快冷至1050~950℃预定温度保温的热模拟方法,确定了中间坯冷却过程中的晶粒尺寸变化规律,提出了中厚板冷却过程中晶粒长大的控制方法,建立了Q345B钢和含Nb-Ti钢在中间冷却过程中的晶粒长大模型.在中间冷却过程中,Q345B钢晶粒稳定性较差,而含Nb-Ti钢晶粒稳定性良好,归因于以铌为主的析出相对奥氏体晶界的钉扎作用.中间坯的强制冷却可控制奥氏体晶粒长大,63mm厚中间坯强制冷却可有效减小平均晶粒尺寸约20μm.在实际生产中,经中间强制冷却后16 mm厚度Q345B钢板的冲击韧性提高25%~70%. 相似文献
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Rian Dippenaar Christian Bernhard Siegfried Schider Gerhard Wieser 《Metallurgical and Materials Transactions B》2014,45(2):409-418
Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, delta-ferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains. 相似文献
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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
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High cycle fatigue properties of gas-carburized 4140 steel were assessed to compare with those of 8620 steel which is widely
used as a carburizing steel. Fatigue limit was evaluated associated with microstructure, case depth, and distribution of retained
austenite and compressive residual stress near the surface. Test results indicated that the reheat quenching method of 4140
and 8620 steels produced a reduction in grain size, retained austenite level, and compressive residual stress at the surface
and an increase in fatigue limit. The fatigue limit of direct-quenched 4140 steel shows substantially lower value than that
of direct-quenched 8620 steel due to larger grain size of direct-quenched 4140 steel. However, the fatigue limit of reheat-quenched
4140 steel is greatly improved and is comparable to the reheat-quenched 8620 steel. This is attributed to the larger reduction
ratio in grain size and deeper case depth of reheat-quenched 4140 steel as compared to direct-quenched and reheat-quenched
8620 steels. 相似文献
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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
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Austenite-grain growth was investigated in a couple of microalloyed steels, one containing Ti and the other containing Nb, Ti, and V, using different reheating temperatures between 1273 K and 1523 K (1000 °C and 1250 °C). Nature and distribution of microalloy precipitates were quantitatively analyzed before and after reheating. Interdendritic segregation (or microsegregation) during casting can result in an inhomogeneous distribution of microalloy precipitates in the as-cast slabs, which can create austenite grain size variation (even grain size bimodality) after reheating. Ti addition reduced the grain size variation; however, it could not eliminate the grain size bimodality in Nb-containing steel, due to the differential pinning effect of Nb precipitates. A model was proposed for the prediction of austenite grain size variation in reheated steel by combining different models on microsegregation during solidification, thermodynamic stability, and dissolution of microalloy precipitates and austenite grain growth during reheating. 相似文献
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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