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.     

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.     

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.     

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．     

Quantification of macrosegregation in continuously cast steel structures

Macrosegregation was investigated in longitudinal sections of continuously cast, experimental slabs of austenitic stainless steel and carbon-manganese steel. The macro-mapping technique was applied to investigate austenitic stainless steel slab sections. The method was based on electron probe microanalysis that was applied in a step-point mode to locally investigate segregation areas. The same stainless steel specimens and carbon-manganese steel specimens were etched to reveal the segregation structure. The macrosegregation in three carbon-manganese steel slabs was represented by etch-prints. The quantitative investigation of structure was performed directly on the macroetched specimens and on the etch-prints. The morphology of macrostructure was characterised by new parameters: segregation factor M, width of a segregation band W_0.5, centreline segregation intensity C and the heterogeneity index H. The segregation factor M was proportional to the inhomogeneity in segregate size distribution and to the segregate area fraction. W_0.5 was defined as the area where the segregate area fraction is greater or equal to half of the maximum value of measured area fractions. Parameter C equal to a ratio M_max/W_0.5, represented the intensity of centreline segregation. The heterogeneity index H was based on the measurements of mean size of segregates and the mean free path within segregate areas. For a homogeneous structure the heterogeneity index is zero and it increases with growing inhomogeneity of a macrostructure.     

Segregation at halfway cracks in continuously cast steel

The mechanism of hot-short embrittlement in a continuously cast steel was explored through application of Auger electron spectroscopy (AES) and Secondary Ion Mass Spectrometry. The AES results demonstrated that the interdendritic fracture surface of previously cracked specimens were highly oxidized with a relatively high sulfur concentration. These results were confirmed with the ion microprobe. AES results showing sulfur segregation to intact interdendritic interfaces which were free of oxidation, demonstrated that the embrittlement was due to sulfur, not oxygen. A wide band of low level phosphorus segregation corresponding to the interdendritic regions was also detected with the ion microprobe.     

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.     

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.     

低合金钢连轧方坯中心裂纹的分析与研究

介绍了鞍山钢铁集团公司新轧钢股份公司低合金钢连轧方坯的生产工艺及其产品内部质量的检验方法,分析了连轧方坯内部缺陷的状况,确定了低合金钢连轧方坯的内部缺陷主要是中心裂纹,找到了导致连轧方坯内部产生中心裂纹的主要原因,研究了防止和减少中心裂纹的措施和方法,并且进行了生产试验,试验结果表明,这些措施和方法可以有效地防止或减少低合金钢连轧方坯的中心裂纹。     

Inclusions in continuously cast steel slabs - numerical model and validation

A numerical model for liquid steel flow and inclusion transport and separation in continuous casting moulds is presented, which covers several features simultaneously. The free surface at the meniscus is computed; the submerged entry nozzle is discretised in detail to resolve the outlet geometry; gas injection through the SEN and its effect on liquid steel flow are considered; capture of inclusions within gas bubbles is covered in a simplified approach; the solidifying shell and the process of inclusion entrapment by the growing shell are simulated. Computed flow profiles are compared to experimental results on water and to measurements on liquid steel in a slab caster. Predicted inclusion profiles in solid slabs are compared to empirical findings from literature. Results of the model for various cases will be presented in a subsequent publication.     

The 3-dimensional structure of macrosegregation in continuously cast high-carbon steel

Three-dimensional segregation maps were obtained by serially sectioning rolled billets of high carbon steel. The sections were macroetched to reveal phosphorus segregation then digitised and combined using a computer. Volume rendering software was used to create images that represented views of continuously cast blooms with unsegregated steel transparent and macrosegregation opaque. The images showed the overall V-structure of macrosegregation and channel like morphology of individual macrosegregates. Samples with a fully columnar structure and a mixed columnar-equiaxed structure were compared. A large equiaxed zone disperses the macrosegregation over a wider area and increases the angle and length of channels. The observed structures suggest that macrosegregation forms through a combination of internal hot tearing and erosion of solid by flowing segregated liquid.     

Fractal analysis of the surface cracks on continuously cast steel slabs

Data concerning the length of longitudinal cracks on the surface of continuously cast steel slabs were collected from two plants. The data were analyzed to find the relation between crack length and crack frequency. The analysis revealed the following.

帘线钢连铸坯中心偏析的控制技术

分析帘线钢连铸过程中中心偏析的形成原理及对铸坯质量的影响。研究帘线钢连铸坯中心偏析的控制措施,包括连铸拉速与钢水温度优化控制、结晶器电磁搅拌、凝固末端轻压下、二冷动态控制等核心工艺技术。通过应用这些技术,帘线钢WLX72A和WLX82A连铸坯平均中心碳偏析指数分别控制在1.042和1.027,综合合格率达到99.97%。     

Some effects of electromagnetically induced fluid flow on macrosegregation in continuously cast steel

Electromagnetically induced and controlled flow of the molten metal inside the solid shell has been found to be a technique for controlling solidification structures and centerline segregation in continuously cast steel billets. Measurements of centerline segregation in steel billets solidified under induced flow indicate a reduction of centerline segregation by a factor of two for carbon, phosphorus, sulfur, chromium, and nickel, and somewhat smaller improvements for other elements. The type of induction stirring technique applied was found to have a substantial effect on the macrosegregation patterns found in the billets solidified under induced flow.     

A study of shell growth irregularities in continuously cast 310S stainless steel

Abstract

Growth irregularities in continuous casting are believed to be associated with crack formation and breakouts. Differential thermal analysis on 310S stainless steel samples indicated primary precipitations of both austenite and ferrite during solidification. In tensile tests on solidifying samples, abrupt shrinkages in volume were detected in the peritectic range of temperatures. Micrographic and microsegregation analysis on samples extracted from a breakout shell revealed high ratios of primary-precipitated austenite in the thick sections of the shell, and high ratios of primary-precipitated ferrite in the thin sections. Alternating precipitations of austenite and ferrite are proposed to occur during solidification. Regions of the shell with high ratios of primary austenite remain in contact with the mould and exhibit high growth rates, whereas regions with high ratios of primary ferrite shrink in volume due to the ferrite to austenite transformation, which results in the formation of air gaps between the shell and the mould and reductions in growth rate.     

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.