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.     

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.     

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.     

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.     

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.     

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。     

连铸异型坯凝固过程的数值模拟

为了解决连铸异形坯的表面及内部裂纹问题,以实测异形坯连铸工艺参数作边界条件,采用有限元方法,利用ANSYS商业软件对铸坯的凝固发展过程进行了数值模拟,模拟结果与实测铸坯表面温度吻合.计算结果表明,对于SS400异型坯,在当前的工艺条件下,仅有圆角处的表面温度在二冷区中前段,温度处于高温塑性区,铸坯的其他部位表面温度均落在相应钢种的低温脆性区.因此二冷区可以进一步采用弱冷方式,使异型坯在二冷段和矫直辊前的表面温度处于高温塑性区.     

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.     

Isotherm velocity in the axial zone of a continuously cast steel ingot

The behavior of the isotherm velocity in a continuously cast solidifying steel ingot is analyzed. An expression is obtained to describe the accelerated motion of isotherms near the thermal center of a cooled body (the center of symmetry of the body) without a phase transition. In the axial zone of the solidifying ingot, where the thermal overheating is insignificant, the isotherm velocity is caused by the action of two opposing factors: (i) the accelerated motion of isotherms that is characteristic of bodies cooled without a phase transition when the thermal center is approached and (ii) the release of the heat of phase transition. As a result, in the axial zone of an ingot made of a low-carbon steel (the initial carbon concentration C ₀ ≤ 0.2%), the liquidus isotherm velocity is almost constant, whereas the isotherm motion at the end of solidification is sharply accelerated, as in the case of purely thermal cooling. For high-carbon steels (C ₀ ≥ 1.0% C), the liquidus isotherm velocity increases, and the velocity of the isotherm at the end of solidification is constant (the effect of the eutectic transformation manifests itself). As a consequence, in low-carbon steels, the pool calculated from the liquidus and pouring-boundary isotherms has the shape of an acute-angled wedge, and the pool calculated from the isotherm of the end of solidification has a rounded shape. In contrast, in high-carbon steels, the pool shape calculated from the liquidus and pouring-boundary isotherms is rounded, and the pool shape calculated from the isotherm of the end of solidification is wedgelike. As a result of the analysis, a mathematical procedure is proposed for the calculation of the isotherm velocity in the two-phase zone and the shape and position of the pool bottom (from the corresponding isotherms) in a continuously cast solidifying steel ingot.     

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.

汽车用钢连铸坯的高温力学性能

运用经典热模拟的方法研究石钢电炉生产汽车用钢的连铸坯高温力学性能,获得了五个钢种热塑性曲线和强度曲线．针对不同方向铸坯取样的结果与其他研究结果作了对比．运用扫描电镜对试样拉断后的断口形貌进行观察,得出了相应钢在各温度区域的断裂机理．讨论了连铸坯质量与高温力学性能的关系．