轻压下条件下管线钢铸坯凝固组织与中心偏析的关系

 为研究在动态轻压下条件下等轴晶和柱状晶组织与管线钢板坯中心偏析的关系,试验通过改变中包钢水过热度来获得铸坯中心不同凝固组织,并分析不同凝固组织内的中心偏析情况。试验铸坯分别采用热酸腐蚀、化学分析的方法,定性、定量分析过热度在11~32℃之间,铸坯厚度方向上元素含量分布和不同凝固组织中的偏析形貌。结果显示：过热度在14℃以下的坯样中心为等轴晶组织,过热度在20℃以上的坯样中心完全为柱状晶组织;在现有轻压下条件下,中心为等轴晶和过热度为32℃时中心柱状晶组织的坯样中心偏析度均较低,但因铸坯中心整个等轴晶区内会分布着偏析点,因而较之中心柱状晶组织,中心为等轴晶组织中点状偏析分布范围较宽;当中心为柱状晶组织时,适当提高过热度、优化压下量,可降低中心偏析度,提高中心偏析评级。     

连铸板坯中心线偏析的预测

从生产经验和文献可知,结晶过程中产生的凝固收缩会导致中心线偏析,很多情况下,在热轧产品的中心也会发现结晶过程中产生的微细缩孔。从这方面讲,板坯凝固过程中形成缩孔簇的尺寸大小,以及形成这种缩孔的条件是首要的,在全面分析统计数据的基础上,对此进行了讨论,得出一个重要结论:当固液比率达到一定的值后,在此部位的铸坯中心只有糊状物,实际上没有更多的液体可以补充。建立了数学模型,用于计算板坯中心部位的流动速度和方向以及预测的疏松。计算表明,在给定的成分和冷却条件下,中心线偏析主要受支承辊的设置、形变和偏心度的影响,同时支承辊之间铸坯的鼓肚也是重要影响因素之一。     

连铸厚板坯轻压下技术的应用

 对首秦2号板坯连铸机轻压下工艺进行了细致的研究。通过连铸坯射钉试验对首秦2号连铸机二级冷却模型进行了校验;通过不同轻压下率试验条件下连铸坯中心偏析的程度确定了合理的、能够有效改善连铸坯中心偏析的轻压下率;通过连铸坯厚度方向不同部位碳硫元素的分析对轻压下工艺改善前后连铸坯中心偏析度进行了对比。结果表明：首秦2号连铸机二级冷却模型能够准确反映连铸坯凝固末端的位置。在拉速为0.70m/min的连铸工艺条件下,320mm厚连铸坯轻压下段为第9和第10段,合理的轻压下率应该保持在0.85～1.0mm/m之间。采用合理的轻压下工艺后,连铸坯中心偏析得到了明显改善,满足了首秦高品质中厚板对连铸坯内部质量的严格要求。     

连铸板坯中心线偏析形成机制与轻压下技术

针对连铸板坯中心线偏析,利用原位分析技术对中心线区域进行扫描。发现连铸板坯中心线区域的宏观偏析由正偏析带和负偏析带共同组成,其中碳、磷、硫这3种元素在中心线正偏析带周围有明显的负偏析带。对这种其周围存在负偏析带的板坯中心线偏析提出了相应的形成机制。最后根据形成机制近似推断了轻压下技术中压下速率与临界固相率这2个重要参数...     

Influencing Factors on Center Segregation in Continuously Cast Pipeline Steel Slabs

The factors which influence center segregation of continuously cast slabs are obtained through analyzing the database of macrostructrue. It indicates that center segregation becomes severer with the increasing content of carbon, phosphorus and sulfur; Center segregation will be reduced obviously if the content of manganese is higher than 1.5% and the ratio of Mn/S is higher than 300; High degree of superheat , high casting speed and increasing width of slabs will increase the degree of center segregation. Since center segregation below class B has little effect on the property of steel, hence, in order to low the ratio of center segregation of class B-1.0 down to 10%, several control strategies are presented as follows: the contents of carbon, phosphorus and sulfur must be focalized in lower range of steel grade demanded, but manganese upper operating range control. The referenced contents of the elements in molten steel are required like this: [C]<0.07%,[P]<0.01%,[S]<0.005%,[Mn]>1.5%,[Mn]/[S]>300; The degree of superheat should be lower than 24℃ and the casting speed should be defined to 1.0-1.1 m·min-1. At the same time, proper secondary cooling system of water distribution should be developed and the precision of continuous caster should be also improved.     

Investigation of Centreline Segregation and Centreline Porosity in CC ‐ Slabs

Improvements of solidification processing in conventional or near net shape casting depend on sophisticated methods of macroscopic examination of central unsoundness and inhomogeneity of the cast material. As long as remarkable deviations referring to the quality features exist between the slab centre and the bulk material such methods should be looked after. Two semi‐macroscopic methods of segregation analysis are discussed in this paper: firstly, the potential of the emission spectral analysis combined with sectioning of a sample and secondly, the computer aided micro‐probe analysis. Both methods are restricted to small local areas. The proneness of elements and steel grades with respect to micro‐ and macrosegregation as well as distribution‐ and segregation coefficients are determined. The effect of a diffusion anneal on homogenisation of segregation is studied. Last not least the precipitation of sulphides, phosphides or carbonitrides is estimated. Ultrasonic detection of the central unsoundness of conventional slabs or thin slabs is a new approach towards a quantitative macroscopic examination. The sample size is 400 mm x 300 mm x 30 mm. To achieve a low ultrasonic noise using a 5 MHz‐transducer the samples are heat treated for grain refinement. “Dog bone porosity”, macroscopic topographical misfit between the upper and lower solidifying shells, periodical corner cracks or systematic transverse centre cracks are detected. Obviously, segregations do not reflect the ultrasonic beam. However, it can be presumed that an interrelation exists between porosity and segregation, although these might be locally apart from each other. This new ultrasonic test of the central unsoundness of continuous cast (c.c.) slabs gives valuable arguments to machine builders and maintenance people to decide on the right concepts for strand guidance, support rollers and secondary cooling. Soft reduction can be optimized and variations in casting speed counteracted by dynamic means. R&D service of this kind can help steelmakers when decisions are impending to build new casting machines or revamp old ones.     

降低高碳钢连铸小方坯中心偏析的研究

通过建立连铸小方坯凝同传热的数学模型，对实际生产中的小方坯凝固状况进行了模拟，发现原配水条件下铸坯表面温度波动大，并针对生产中连铸坯中心碳偏析较大的问题，对二冷水进行了优化。优化配水后的工业试验结果表明，通过凝固模型计算优化后的二冷配水可以降低中心碳偏析，提高连铸坯低倍质量。     

Micro‐Segregation Behavior of Solute Elements in the Mushy Zone of Continuous Casting Wide‐Thick Slab

Based on the assumption that the transverse cross‐section of dendrites is a regular hexagon, a coupled macro‐heat transfer and solute diffusion model was developed to describe micro‐segregation behavior of solute elements in the mushy zone during wide‐thick slab continuous casting. The heat transfer model and the micro‐segregation model were validated by the industrial tests and the experiment measurements in literatures, respectively. The results show that the cooling rate decreases from 51.1°C s^?1 at the surface to 0.13°C s^?1 at the center of the wide‐thick slab of peritectic steel continuous casting. The effective cooling rate equaling to 1.78°C s^?1 mainly depends on mold cooling and varies little with casting speed. The micro‐segregation behavior of solute elements in the mushy zone depends on back diffusion ability and local equilibrium at interfaces. Compared with other elements in the steel, phosphorus, and sulfur elements exhibit a much higher segregation ratio at the end of the solidification and are more significantly affected by the initial carbon content.     

Results of Research into the Segregation Behaviour of Manganese,Silicon and Chromium in Continuous Casting

For different steel grades the segregation factors of several alloying elements, both for crystal and for centre segregation, have been determined using electron probe microanalysis (EPMA). With this method two‐dimensional quantitative concentration images for several elements are assigned. The segregation factors depend on the chemical composition of a steel grade, in particular on its carbon content. Within a category of steel grades, the segregation factor will increase with increasing carbon content. For the first time, a large number of tests were performed on 53 slab samples from different steel grades to establish the segregation factors. The effects of the slab thickness, soft reduction and carbon content on crystal and centre segregation have been investigated. The results are essential for modelling the segregation phenomena occurring at solidification.     

钢水凝固收缩与板坯铸机开口度参数设计

对板坯连铸机钢水凝固收缩特点及开口度参数对铸机生产的影响、连铸机开口度参数计算的理论依据进行了系统的分析,济钢板坯连铸机开口度设计生产实践表明,以铸机工况条件、钢种拉速及铸坯实物质量控制为基础,理论结合实践来设计铸机开口度参数的方法是合理实用的,铸坯的内部质量缺陷,特别是中间裂纹、中心偏析控制在了较低水平。     

不同拉速工艺下连铸板坯横截面中心偏析和夹杂物的原位统计分布分析

采用原位统计分布分析技术对不同拉速工艺条件下连铸板坯中心横截面的偏析特性和夹杂物分布进行了研究。通过对3.0 m/min和4.2 m/min拉速条件下C,Si,Mn,P,S及Al系夹杂物含量在连铸板坯中心横截面的分布规律对比,发现在低拉速工艺条件下,样品中C,Mn,P等元素在连铸板坯横截面的统计均匀性好于高拉速工艺条件下样品,Si,S等元素则呈现相反的规律,S,P,C等元素易形成“点状”偏析,呈不连续分散分布的特点,Al系夹杂物分布均匀,粒度小;高拉速工艺条件下,样品中S,P,C等元素易形成“带状”偏析,     

连铸坯凝固过程中传输现象基础知识系列讲座 第六讲 连铸坯凝固过程的溶质偏析(一)

连铸坯在凝固过程中产生的偏析对连铸坯质量影响很大，其诱发因素是溶质交换，国内外专家采用数学模型对各种状态下的溶质再分配作了定量方面的研究，已引起广泛重视。     

On the Formation of Centreline Segregation in Continuous Slab Casting of Steel due to Bulging and/or Feeding

Centreline macrosegregation is often observed in continuous slab casting of steel. Two of the main macrosegregation formation mechanisms are bulging and feeding. Both were studied and compared in the current work by using a two‐phase volume averaging model considering only columnar solidification. The casting of the strand itself is modelled by applying a predefined velocity following the casting speed and solid shell deformation (e.g. bulging). Three different cases are simulated and discussed. (i) The first case considers the influence of the feeding flow during solidification without taking bulging into account. Negative macrosegregation is observed in the centre of the casting in this case. (ii) The second case takes the flow caused by series of bulging along the solidifying strand shell into account, and is, therefore, representative for an ideal situation where bulging takes place without solidification shrinkage. In this case positive centreline segregation is found. (iii) The last case shows the results of a simulation which combines both shrinkage‐ and bulging‐induced flows. It is found that under the current casting conditions the bulging effect dominates over the shrinkage effect, and so positive centreline segregation is predicted.     

动态轻压下在中碳钢铸坯中心偏析控制中的应用

介绍了某厂直弧型板坯连铸机生产中碳钢过程中,通过对动态轻压下技术参数进行优化,完善了动态连铸控制工艺,使中碳钢连铸坯中心偏析得到明显改善,B级偏析率平均由36．84％下降到15．91％。     

A Kinematic Thermal Model for Descending Slabs with Velocity Boundary Layers:A Case Study for the Tonga Subducting Slab

For the purpose of investigating the influence of metastable olivine(MO)phase transformations on both deep seismicity and stagnation of slabs, we constructed a 2-dimensional finite element thermal model for a 120 Ma-old 500 dipping oceanic lithosphere descending at 10 cm/yr with velocity boundary layers, which would mitigate the interference of constant velocity field for the slab.The resulting temperatures show that most of intermediate and deep earthquakes occurring within the Tonga slab are occurring inside the 8000C and 12000C isotherm, respectively. The elevation of olivine transformation near～410 km and respective persistence of metastable olivine and spinel within the transition zone and beneath 660 km would thus result in bimodal positive, zonal, negative density anomalies, respectively. These results together with the resulting pressure anomalies may reflect the stress pattern of the Tonga slab:(i)slab pull force exerts above a depth of～230 km;(ii)MO existence changes the buoyancy force within the transition zone and facilitates slab stagnation at a depth of 660km;(iii)as the subducting materials accumulated over 660 km, deepest earthquakes occur due to MO transformation;(iv)a flattened'slab' may penetrate into the lower mantle due to the density increment of Sp transformation.     

影响退火钢板表层Mn析聚的因素及提高2030钢板抗锈蚀能力的对策

从2030与1420两冷轧单元工况上的差异出发,考察了2030CAPL和2030BAF中影响Mn等易氧化元素在钢板表面富集的因素.结果表明,退火温度、退火时间、退火前清洗介质中含Si与否、钢卷冷轧时的变形量等对Mn等的析聚有较大的影响;宝钢2030冷轧板表面Mn等元素的过度析聚也归因于退火温度(过高)、退火时间(过长)、CAPL退火前清洗介质中含有Si、钢卷冷轧时的变形量与退火温度时间的不完全匹配等的共同作用.考虑到2030冷轧单元对这些关键因素进行严格地控制和匹配较为困难,提出了"退火后轻微酸洗处理"的提高2030冷轧板抗锈蚀能力的措施.