Thermo-elasto-visco-plastic finite element analysis on formation and propagation of off-corner subsurface cracks in bloom continuous casting

The formation and propagation of the popular off-corner subsurface cracks in bloom continuous casting were investigated through thermo-mechanical analysis using three coupled thermo-mechanical models.A two-dimensional thermo-elasto-visco-plastic finite element model was developed to predict the mould gap evolution,temperature profiles and deformation behavior of the solidified shell in the mould region.Then,a three-dimensional model was adopted to calculate the shell growth,temperature history and the development of stresses and strains of the shell in the following secondary cooling zones.Finally,another three-dimensional model was used to analyze the stress distributions in the straightening region.The results showed that the off-corner cracks in the shell originated from the mould owing to the tensile strain developed in the crack sensitive regions of the solidification front,and they could be driven deeper by the possible severe surface temperature rebound and the extensive tensile stress in the secondary cooling zone,especially upon the straightening operation of the bloom casting.It is revealed that more homogenous shell temperature and thickness can be obtained through optimization of mould corner radius,casting speed and secondary cooling scheme,which help to decrease stress and strain concentration and therefore prevent the initiation of the cracks.     

Analysis of thermomechanical behaviour in billet casting with different mould corner radii

Abstract

A finite element thermal stress model to compute the thermomechanical state of the solidifying shell during continuous casting of steel in a square billet casting mould has been applied to investigate longitudinal cracks. A two-dimensional thermoelastoviscoplastic analysis was carried out within a horizontal slice of the solidifying strand which moves vertically within and just below the mould. The model calculates the temperature distributions, the stresses, the strains in the solidifying shell, and the intermittent air gap between the casting mould and the solidifying strand. Model predictions were verified with both an analytical solution and a plant trial. The model was then applied to study the effect of mould corner radius on longitudinal crack formation for casting in a typical 0·75%/m tapered mould with both oil and mould powder lubrication. With this inadequate linear taper, a gap forms between the shell and the mould in the corner region. As the corner radius of the billet increases from 4 to 15 mm, this gap spreads further around the corner towards the centre of the strand and becomes larger. This leads to more temperature non-uniformity around the billet perimeter as solidification proceeds. Longitudinal corner surface cracks are predicted to form only in the large corner radius billet, owing to tension in the hotter and thinner shell along the corner during solidification in the mould. Off corner internal cracks form more readily in the small corner radius billet. They are caused by bulging below the mould, which bends the thin, weak shell around the corner, creating tensile strain on the solidification front where these longitudinal cracks are ultimately observed.     

Three-dimensional FEM study of fluid flow in mould for beam blank continuous casting: influence of straight through conduit type SEN

Abstract

In order to increase the beam blank cleanliness, the aim of this work is to analyse the flow field in the mould of beam blank continuous casting, to find the factors influencing the strand cleanliness and then to optimise the process parameters. A three-dimensional steady finite element model was developed to simulate and analyse the turbulent flow field in the mould. The volume of fluid model was used to track the free surface evolution at the meniscus. The influences of processing parameters, such as casting speed and nozzle parameters, on the molten steel flow in the strand (such as vortex location, liquid steel impact depth, velocity and fluctuation of the liquid steel at free surface) were analysed and the optimum processing parameters determined based on mass calculation. The results of this research project have been applied in actual production, and it has been shown that they are very useful and efficient for improving the steel cleanliness and controlling the surface cracks on the beam blank web.     

连铸矫直过程板坯角部横向裂纹数值分析

 连铸过程中板坯角部可能出现横向裂纹,这些裂纹严重影响着连铸坯的质量和产量。本文采用显式动力学有限元方法对连铸矫直阶段连铸板坯表层金属应力变化规律进行了模拟,分析了角部应力的变化规律,并对矫直过程中连铸板坯应力分布对角部横向裂纹的影响进行了讨论。研究结果对指导连铸生产具有重要的实际意义和理论意义。     

Application of high-basicity mould fluxes for continuous casting of large steel slabs

Controlling the formation of longitudinal cracks on hypo-peritectic steel slab surfaces is one of the key challenges in continuous casting worldwide. Based on the production in Chongqing Iron & Steel Co., mould fluxes with high basicity ranging from 1.7 to 1.8 were put forward and the effects of compositions on the basic properties of mould flux were studied in the present paper. After laboratory experiments, a high-basicity mould flux with an increased crystallisation speed to abate the heat-transfer capability in the meniscus area was fully applied in the first steelmaking plant of Chongqing Iron & Steel Co. Using the high-basicity mould flux, for more than 3 million tonnes of slab production, the large longitudinal crack was eliminated and the rate of formation of small longitudinal cracks decreased greatly. During the production process, sticking that impedes the smooth running of continuous casting was infrequent, and the spot check for longitudinal cracks on micro alloy steel surface was simplified in the slab finishing process. Furthermore, since the application effect of mass production is remarkable, the hot charging process is better implemented, and the contract can be accomplished in time. In summary, the problem of longitudinal cracks on hypo-peritectic steel slab surfaces is finally resolved and the high-basicity mould fluxes have become indispensable auxiliary materials during continuous casting of hypo-peritectic steel.     

Two-dimensional heat transfer model for secondary cooling of continuously cast beam blanks

Abstract

A two-dimensional heat transfer model was developed for the secondary cooling system during beam blank continuous casting. The finite element method was used to calculate the heat transfer. Accurate cooling boundary conditions in the secondary cooling zone are involved, including spray water cooling, water evaporation cooling, radiation cooling and roll contact cooling in the casting direction and non-uniform distribution of spray water flow density in the cross-section. The causes of longitudinal crack at the fillet during Q235 steel continuous casting were analysed on the basis of the simulation of the developed model, and then the spray water flow and the transverse nozzle layout were optimised. Practical results show that the surface quality of the beam blank improved after optimisations. Numerical results from the present model were validated using previous experimental measurements, which show good agreement.     

粗轧过程轧件表面裂纹演变行为热力耦合有限元分析

 连铸坯可能出现表面裂纹,其在轧制过程中的演变行为严重影响着轧制产品的质量。本文采用热力耦合有限元方法对轧制过程轧件表面裂纹演变行为进行了分析。获取了轧制过程中轧制变形区内轧件表面裂纹形状变化规律、裂纹附近区域应力场和温度场分布场分布情况。计算结果表明在轧制入口区域,裂纹逐渐闭合,然而,在轧制出口区域,裂纹又重新扩展开,裂纹尖端处呈现拉应力状态。同时,发现变形区内裂纹尖端发生温度成双峰变化规律。     

微合金钢连铸坯角横裂的研究进展

针对微合金钢连铸生产过程中铸坯出现的角部横裂纹缺陷,从其产生原因、影响因素及解决方法等方面进行阐述。介绍了角横裂产生于结晶器内,并进一步扩展于二冷区。分析了连铸过程中的应力、热塑性、结晶器锥度和二次冷却模式等因素对微合金钢连铸坯角横裂的影响。总结了目前解决角横裂的几种方法。重点介绍了连铸微合金钢的脆化机制、微观组织和微合金元素对连铸坯热塑性的影响。最后对微合金钢连铸坯角横裂新的解决方法进行了展望。     

Analysis on morphology and stress concentration in continuous casting bloom to learn the formation and propagation of internal cracks induced by soft reduction technology

ABSTRACT

The crack formation and propagation were analysed according to actual internal crack morphology and the finite element model simulation of stress concentration. The results showed that most cracks were distributed along the transverse and the longitudinal section of the bloom. Distinct differences between these two major types of cracks were found in dimension and inclination, which were owing to different local stress concentrations. In the longitudinal section of the bloom, shear stress was concentrated in the brittle temperature region, which led to the formation of initial cracks and subsequently cracks propagation. Meanwhile, the maximum tensile stress occurred at the edge between the brittle temperature region and surrounding material, which resulted in crack formation and propagation along the transverse section of the bloom. This phenomenon was due to the obvious bulging deformation of the solidified shell induced by soft reduction.     

薄板坯表面纵裂形成机理及结晶器锥度研究

从力学角度出发对薄板坯表面纵裂纹的形成机理进行了探讨,分析了结晶器锥度控制工艺对薄板坯表面纵裂纹的影响,详细论述了纵裂纹的控制措施。研究结果表明,局部的拉应力集中是铸坯表面纵裂纹形成的主要原因之一。表面纵裂纹的控制措施包括2方面,一方面是通过优化连铸工艺提高凝固壳生长的均匀性,另一方面是通过优化结晶器内腔结构和调控锥度来促进结晶器与凝固壳接触状态的最佳化。     

非调质钢38MnVS轧制裂纹机制分析

 针对非调质钢38MnVS轧制过程中出现的裂纹,采用金相显微镜、扫描电子显微镜、小样电解技术对裂纹区域进行二维、三维分析。结果表明,裂纹两侧存在明显脱碳层,在裂纹周边及延伸区域存在大量沿线分布的硅锰酸盐夹杂物。由于大量长条和不规则状硅锰酸盐夹杂物的存在,在连铸过程铸坯表面产生了微细的纵向裂纹。在铸坯热送过程,微细裂纹进一步扩展,形成贯穿裂纹,从而导致整个轧材报废。根据裂纹产生的机制,优化连铸过程保护渣,取消对铸坯的热装热送,能有效控制裂纹产生,使成材率明显提升。     

Q355R连铸坯表面微裂纹研究

摘要：裂纹是影响连铸坯质量的主要缺陷,其成因和影响因素复杂众多。针对Q355R连铸坯表面微裂纹,在铸坯典型裂纹处采用金相观察、扫描电镜、能谱进行表征,进一步分析其形成机制,进而对连铸工艺进行优化改进。结果表明,铸坯表面的微小裂纹,其形貌不同于传统的网状星形裂纹和横裂纹,裂纹端主要为钝口且无延伸端,个别裂纹上端存在微裂纹细线,裂纹端呈“针”状且有延伸段。微裂纹主要成因是由结晶器卷渣、钢水二次氧化、钢水过热度大以及在连铸机弯曲和矫直区域外部应力共同作用所导致。通过对水口结构优化,改善结晶器流场,减弱结晶器卷渣,以及对连铸工艺优化,使铸坯表面微裂纹得到了有效控制。     

基于XFEM的岩体卸荷过程裂纹起裂扩展规律研究

地下和边坡工程开挖常涉及岩体卸荷问题,采用ABAQUS软件中的扩展有限单元法(extended finite element method,XFEM)对开挖卸荷过程岩体内部裂纹的起裂扩展进行了模拟,通过计算裂纹尖端应力强度因子研究了其起裂特征,并探讨了起裂影响因素,通过记录裂纹扩展形态研究了其动态演化模式.结果表明,卸荷过程中卸荷速率越快,裂纹长度越长,倾角越大,其起裂越容易;并且裂纹面受到的正应力不断减小,剪应力不断增大,裂纹扩展主要由剪应力控制,这与理论分析结果一致.裂纹最终扩展演化形态也与物理试验相近,充分表明运用扩展有限单元法研究岩体裂纹问题的可靠性.      

45K-DP钢板坯星状裂纹产生原因探析

主要描述了宝钢4号连铸机生产45K-DP板坯产生表面星状裂纹的形貌,探讨了星状裂纹发生原因以及针对预防星状裂纹所采取的措施。通过对星状裂纹金相组织和扫描电镜分析发现,45K-DP钢星状裂纹起始于连铸坯发生相变之前,并且在星状裂纹位置发现含有Cu的成分。通过加强4号连铸机结晶器铜板寿命管理、提高铸机扇形段驱动辊轴承座安装精度,杜绝了45K-DP钢星状裂纹的发生。     

条形药包爆炸全场应变以及裂纹动态断裂特性研究

为了探究不同起爆位置下条形药包全场应变以及裂纹动态断裂特性,采用爆炸荷载动态焦散线实验系统和数字图像相关方法（DIC）,开展了爆破模型实验研究。研究结果表明:条形药包一端起爆时,起爆点处翼裂纹扩展长度最小,随着炸药爆轰的传播,翼裂纹扩展长度增长;中心起爆时,中心位置翼裂纹扩展长度小于两端位置翼裂纹扩展长度,一端起爆时非起爆端翼裂纹扩展长度最长。无论中心起爆或一端起爆,条形药包中心区域翼裂纹扩展主要为Ⅰ型裂纹,并且中心翼裂纹起裂韧度最大,端部翼裂纹为以Ⅱ型为主的Ⅰ?Ⅱ型复合裂纹。一端起爆时,拉压应变作用范围沿炸药传爆方向传递,且非起爆端拉压应变作用区域大于起爆端,压应变最大值为距起爆点约0.67 ~ 0.83倍的装药长度。中心起爆时,拉压应变的作用过程沿起爆中心向两端呈对称形式传播,中心点位置应变最大。两种起爆方式下都出现端部压应力集中现象。      

减少连铸异型坯表面纵裂纹的工艺研究

马钢连铸异型坯存在表面纵裂纹,是造成轧后废品的主要原因。对连铸坯表面温度测量,对结晶器传热进行热模拟分析。结果表明,连铸坯表面温度差异很大,由于异型坯形状的特殊性,结晶器传热存在严重的不均匀性,而热应力导致纵裂纹。为了抑制表面总裂纹,必须改善结晶器传热,改进保护渣性能,稳定浇注条件。     

连铸板坯表面纵裂纹预判系统的开发

采用SQL Server 2008数据库技术,对连铸生产过程中板坯表面纵裂的影响因素进行了回归分析,模拟了实际生产线上的数据采集与提取,建立了连铸板坯表面纵裂纹预判系统模型,基于VS2010系统开发平台,使用C#语言+SQL SERVER 2008数据库开发了连铸板坯表面纵裂纹铸坯预判系统。     

FTSC薄板坯连铸中碳钢保护渣研究及开发

针对唐钢FTSC薄板坯连铸所产生的板坯裂纹、表面夹渣、卷渣及漏钢现象进行保护渣生产试验研究与理论分析。研究结果表明,组分变化对保护渣熔化温度和粘度等指标有着重要的影响。通过实验室对保护渣组份及其变化对其性能影响的研究,结合唐钢FTSC薄板坯连铸自身特点,设计出保护渣基本配方,且在不断试验改进中,最终开发出适合唐钢薄板坯连铸用中碳钢保护渣C2.     

倒角结晶器在涟钢板坯连铸生产中的应用

 为了消除微合金化钢连铸板坯的角部裂纹缺陷,使得板坯在不进行离线切角的条件下满足轧制要求,采用倒角结晶器技术进行了大量的工业试验,分析并解决了倒角型板坯存在的角纵裂及结疤问题,实现了该技术的规模化连续生产。工业应用结果表明,倒角结晶器技术不仅能够控制板坯的角横裂缺陷,而且能够大幅度降低超低碳钢热轧板卷边直裂的发生率,因此可为钢铁企业带来巨大的经济效益。     

连铸中碳钢用CK—2保护渣的研制

中碳钢亦称裂纹敏感性钢,在加铸过程中容易产生铸坯表面纵裂纹等缺陷,使用性能合适的保护渣可以改善结晶器内初始坯壳的传热条件,控制表面裂纹的产生,介绍了研制武钢三炼钢厂中碳钢用保护渣的过程及裂纹控制效果,分析了保护渣使用过程的性能变化和渣膜微观结构。