Evaluation of internal and subsurface quality of continuously cast billets and slabs by ultrasonic techniques

Abstract

The paper highlights the developments made in ultrasonic techniques at R&D and SS Division of Tata Steel Ltd in detecting, measuring and analysing the continuously cast billets/slabs with respect to its internal as well as subsurface quality. Different continuously cast billets and slabs could be ranked according to their cleanliness levels even with the conventional ultrasonic flaw detector. The effectiveness of ladle furnace treatment and vacuum degassing (RH degassing) in improving cleanliness level in slabs of extra deep drawing grade and that of electromagnetic stirring in low and high carbon billets, could be successfully assessed using these techniques. In order to convert the down graded first and last slabs of a sequence of auto grade interstial free into the prime grade slabs, the scarfing depth and cutting length were optimised based on the ultrasonic results. Some results using a computer controlled ultrasonic multiscan imaging system, on continuously cast steel billets, have also been presented.     

The influence of mold behavior on the production of continuously cast steel billets

The origins of rhomboidity, longitudinal corner cracks, and breakouts in the continuous casting of steel billets have been investigated with the aid of heat flow and stress analyses of the mold wall. It has been shown that these problems can be linked to intermittent boiling in the cooling water channel, which may occur asynchronously on different faces of the mold. A mechanism based on asynchronous, intermittent boiling and nonsymmetrical cooling of the mold wall has been formulated which explains the influence of billet size, cooling-water velocity, water pressure, cold face roughness, and steel carbon content on the formation of rhomboidity and longitudinal corner cracks. Prevention of intermittent boiling is thereby shown to be a key factor in the production of defect-free billets. This can be accomplished by raising cooling water velocity, increasing mold wall thickness, increasing water back pressure, or roughening the cold face near the meniscus. These measures should also be effective in reducing the frequency of breakouts beneath the mold.     

Initial development of thermal and stress fields in continuously cast steel billets

A mathematical model has been developed to compute the thermomechanical state of the shell of continuously cast steels in a round billet casting mold. The model determines the temperature distributions, the stresses in and the gap between the casting mold and the solidifying strand. The effect of variations in steel carbon content and mold taper on the thermal, displacement, and stress fields are examined. Comparisons with available experimental observations verify the predictions of the model. The model demonstrates that the thermal shrinkage associated with the phase change from delta-ferrite to austenite in 0.1 Pct C steel accounts for the decreased heat transfer observed in that alloy, as well as its susceptibility to cracking. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

On the formation of pipes and centerline segregates in continuously cast billets

Centerline macrosegregation in continuously cast steel billets, blooms, and slabs is a significant problem. Thermal contraction of the solidified shell at the final end of the liquid pool causes a separation in the central portion of the strand and formation of a pore. When this pore fills with liquid, centerline macrosegregation results. Segregation formation is influenced by the cooling and casting conditions. The effects of those parameters are discussed in this article. Thermal contraction also causes formation of a large pipe in the very last portion of the strand. The casting parameters that control the centerline macrosegregation also control the size of the pipe.     

Surface quality of continuously cast semis

Abstract

This meeting in the informal conference series organised by the Iron and Steel Society of the Institute of Materials, Metals and Mining, held in London on 6–7 November 2006, attracted 64 delegates from 13 countries.

These included representatives from 14 steel producers, three machine builders, six suppliers of equipment and services, four steel company research centres and one university. In total 26 papers were presented.     

Cavity sequences in continuously cast billets: Part II. Stochastic models

A stream of voids is often encountered in the core of continuously cast steel billets. This article presents mathematical simulations of the cavity process. Parameters of the models are deduced from the data described in Part I of this article. A Markov simulation represents the chain as a binary sequence; an autoregressive (AR[2]) model and two other related simulations produce series of interval lengths and void fractions. A bridge model uses two independent random numbers to simulate cavity events. The performance of the models is checked against empirical findings. Dynamics of the cavity process are, finally, discussed.     

Final solidification,centre segregation and precipitation phenomena in continuously cast round billets

Archimedes’ principle was applied to strand lengths of up to 14 m of round 177 mm diameter billets to determine cavity volume V (in cm³/m) for a series of tubemaking steels. Under steady state casting conditions, V depends primarily on the chemical composition and solidification behaviour of the steels. Averages ranged up to in an extreme case (bearing steel; v_C = 2.0 m/min). Stopping the strand and abrupt lowering of withdrawal speed made it possible to artificially induce individual shrinkage cavities with peak values of V = 45 cm³/m. In the round mould, subsequent necking of the liquid crater is initiated in this process. Whisker-like and icicle-like dendrites can be observed in horizontally formed macroscopic shrinkage cavities. The liquid crater is broken off by periodic bridging (the mini-lingot model). As permeability decreases, the bridges deflect downward in a U-shape under the ferrostatic load. The residual inter-dendritic melt is squeezed out in this plastic deformation process. Some bridges ultimately break. This interpretation is based on comprehensive CMA studies and concentration maps. Cores taken from the centre of a strand of bearing steel were submitted to various heat treatments between 1100 and 1350°C. Homogenization occurs below solidus (T < 1225°C). Carbides dissolve and sulfides become globular. Spatial segregation, on the other hand, increases, above solidus. In the extreme case at 1350°C/30 h, the residual melt at the grain boundaries migrates practically completely from the surface zone to the centre of the cylindrical sample (40 mm diameter). The propellent forces for this macro-heterogenization are provided by the activity gradient from the cylindrical surface to the centre and minimization of interface energy γ/I.     

Cavity sequences in continuously cast billets: Part I. Analysis of empirical data

Voids in the core of continuously cast steel billets are accompanied by segregation and therefore can be a problem. Samples of round steel billets are analyzed in this work with respect to the dynamics of central porosity formation. Longitudinal radiographs of the samples have been digitized. The stream of porosities is represented as a time sequence. Statistical properties of the series are determined. Spectral analysis is used to produce autocorrelations and power spectra and, thus, to test for periodicities. Clustering of voids is studied. The distribution functions of interval lengths and void fractions are examined. A U-shaped relation is observed between interval lengths and interval void fractions. Techniques of nonlinear analysis are applied to quantify the degree of randomness and complexity of the signals.     

连铸红坯直轧的工艺实践

介绍宏大钢铁连铸红坯直轧的工艺实践,并对试生产情况作简要分析,提出合理的优化工艺思路.     

Obtaining high-quality cast billets from Cu-Mg alloys

The technology of obtaining cast billets of the Cu-Mg (0.1–1.0%) alloys, in which continuous smelting and continuous horizontal casting are implemented in one smelting-casting aggregate, is developed. In this case, the refinement and homogenizing processing of the melt is performed continuously during the entire time of smelting and casting. Technical solutions accepted for this purpose make it possible to overcome the difficulties in providing the precision magnesium content, eliminate its strong liquation in the melt, neutralize the high surface activity of magnesium and its liability to oxidation, stabilize the recovery of magnesium from the foundry alloy and minimize the contamination of the melt by its oxidation products, attain deep removal of gases and nonmetallic inclusions from the melt, homogenize the melt in regards to the chemical composition, and acquire high-quality cast billets.     

Promising method of producing cast billets from superalloys

A new method is proposed for electroslag casting of products made of superalloys. A melt is modified by ultradispersed refractory particles, and the accumulation of a metal in a melting vessel and its pouring into a mold are performed in an argon atmosphere. The equipment designed for this method is described. The results of comparative analysis of the properties of the cast metal are presented, and the advantages of the method are shown.     

M-EMS对马氏体不锈钢连铸坯质量的影响

根据工业试验结果,分析了结晶器电磁搅拌（M-EMS）对马氏体不锈钢连铸坯的中心疏松、等轴晶率、缩孔及表面质量的影响,并对电磁搅拌对铸坯凝固的影响机理进行了探讨。结果表明：经结晶器电磁搅拌后,当平均磁感应强度B为0．066T时,铸坯中心等轴晶率平均达到了50％,最高达57％,中心疏松均在1.5级以下,中心缩孔90％在1．0级内;铸坯表面质量由使用二冷区电磁搅拌的85％提高到97％,为结晶器电磁搅拌工艺参数的优化及设计提供了一定的依据。     

Effect of oscillation-mark formation on the surface quality of continuously cast steel slabs

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 .     

方坯连铸机浇铸高碳钢

方坯连铸机浇铸的高碳钢坯，其深加工产品是机械性能要求较高的钢丝绳、预应力钢丝、轮胎钢丝及弹簧铜丝等．钢中脆性、大颗粒夹杂物是工艺过程的脱氧产物；高碳钢铸坯凝固过程中产生严重的中心偏析和中心疏松缺陷．这些夹杂物和缺陷使得钢丝在冷拔过程中或在使用过程中出现非疲劳断丝．避免质量事故的方法是：控制高碳钢钢水的质量，将炼钢脱氧过程产生Al2O3脆性氧化物夹杂，通过控制渣系使生成的MnO2—Al2O3—SiO2系三元夹杂分布在低熔点区；必须实现无氧化保护浇注；中间包内腔优化对夹杂物上浮效果显著；保证工艺过程操作的最小温度降低；采用连铸坯轻压下技术或高强度高密度冷却技术、结晶器电磁搅拌和控制冷却强度．     

Control of inclusion characteristics in direct cast steel billets

Two main criteria are used to characterize steel cleanliness. First the number and size of inclusions must be at the minimum level achievable with state-of-the-art steelmaking and casting equipment and procedures. Second the remaining small inclusions must have a chemical composition suitable for the steel application. In high carbon steel destined for wire drawing, inclusions need to be as deformable as possible.In this paper the inclusion behavior during hot rolling continuously cast (c.c.) billets to wire rods is reviewed. The relationship between inclusion deformability and composition was investigated at the laboratory scale and the results were applied to the control of cleanliness in c.c. billets. Production parameters affecting steel cleanliness are discussed.     

连铸坯表面网状裂纹

对连铸坯和热轧厚板表面网状裂纹附近的化学成分及其组织进行了分析,发现裂纹附近存在Cu和Cr元素,裂纹沿着晶界延伸.可见裂纹形成的原因为:结晶器镀铬层磨损导致铜板与连铸坯粘结,液态铜通过奥氏体晶界向铁基体内渗透.富集在奥氏体晶界的铜极大地恶化了钢的塑性是导致裂纹形成的主要原因.在此基础上提出了控制连铸坯和热轧厚板表面网状裂纹的措施.