20钢Φ300 mm圆铸坯中间裂纹成因分析和改善工艺措施

针对20钢φ300mm圆铸坯低倍组织出现中间裂纹的位置和形貌,结合连铸设备和工艺进行分析,得出结晶器内坯壳厚度不均匀,拉矫机矫直压力波动大,铸坯在受到热应力和矫直应力的作用下沿晶界开裂形成中间裂纹。通过改善结晶器软化水和连铸二冷水的质量,钢水过热度由25～35℃降至20～30℃,结晶器电磁搅拌电流由550 A降至350 A,稳定矫直压力,铸坯矫直温度≥950℃,避免了圆铸坯中间裂纹的出现。     

大断面连铸圆坯中心裂纹的成因和控制研究

针对某钢厂Φ600 mm AISI4130钢连铸大圆坯存在的中心裂纹,通过低倍观测到裂纹分布在V型偏析区内,采用原位分析发现中心主要为碳、硫偏析,采用扫描电镜观察裂纹形貌发现裂纹为凝固前沿卵形树枝晶开裂形成的空隙,分析裂纹成因是凝固末期低熔点元素富集芯部,同时凝固收缩使铸坯中心产生较大的拉应力使卵形树枝晶开裂。运用ANSYS软件建立大圆坯传热和应力模型,计算铸坯的温度场和应力场,模拟计算结果表明,凝固末端铸坯中心拉应力超出抗拉强度。提出在凝固末端施加强制冷却工艺,通过模拟计算验证了末端强冷可加速铸坯外部向中心收缩,抑制铸坯中心凝固收缩产生较大的拉应力,有效降低大断面圆坯中心裂纹的产生风险和概率。     

天钢连铸板坯中心裂纹的成因分析

针对天钢炼钢厂在连铸生产中出现的中心裂纹,结合VAI动态辊缝模拟软件、铸坯凝固过程应变理论及天钢炼钢厂的生产实践,分析了产生铸坯中心裂纹的影响因素.在设备方面,辊缝超差,特别是凝固末端辊缝超差是造成中心线裂纹的主要原因.在工艺方面,铸坯在冷却过程中,由钢水成分、浇注温度以及拉速变化引起的相变降低了钢的高温塑性.在外力作用下,坯壳承受的应力之和超过了钢的允许强度和应力时,铸坯就会产生裂纹.     

宽厚板连铸坯鼓肚变形数值计算研究

连铸过程中铸坯已凝固,坯壳在钢水静压力作用下发生鼓肚变形,影响浇铸过程顺行与铸坯质量。以宽厚板连铸坯为对象,采用数值计算方法定量研究了其在连铸过程三个典型铸流位置L1 (弯曲位置)、L2 (弧形段中间位置)、L3 (矫直位置)的鼓肚变形规律。由L1至L3,坯壳鼓肚变形及其导致的凝固前沿拉伸应变均不断增加。凝固前沿厚度方向拉伸应变εxx、拉坯方向拉伸应变εyy与宽度方向拉伸应变εzz呈集中分布趋势,三者分别加剧三角区裂纹、中间裂纹及角部裂纹风险。随着拉速由0.7 m/min增大至0.9 m/min,宽面坯壳鼓肚变形与εxx、εzz先增加后减小,而窄面坯壳鼓肚变形与εyy持续增大。     

连铸板坯中间裂纹成因分析

通过现场数据统计和典型工艺试验,分析了安钢210 mm和230 mm厚板坯内部中间裂纹产生的主要原因。分析认为,造成安钢板坯内部中间裂纹的主要原因是由于铸坯带液芯矫直,凝固前沿受矫直力作用产生裂纹并沿柱状晶晶界扩展,这也解释了安钢210 mm铸坯内部质量好于230 mm的主要原因;同时试验表明低拉速和强配水的工艺可以改善铸坯中间裂纹。     

连铸方坯轻压下内裂纹形成研究

通过Abaqus/Explict建立方坯轻压下热力耦合模型,并通过凝固相变模型计算裂纹敏感性温度区间,研究内裂纹形成机理,得到了轻压下过程方坯温度分布、应力应变状态,分析了轻压下过程中脆性温度区内裂纹与等效塑性应变、最大主应力的关系。研究结果表明,脆性温度区内应变超过临界应变或存在拉应力时内裂纹开始萌生,脆性温度区拉应力是造成裂纹形成的最要因素,分析结果为优化轻压下工艺提供参考,可以优化铸坯轻压下的变形分布,提高铸坯内部质量。     

凝固末端电磁搅拌和轻压下复合技术对大方坯高碳钢偏析和中心缩孔的影响

基于ANSYS软件建立了310 mm×360 mm断面大方坯连铸过程二维凝固传热数学模型,并采用窄面射钉试验及铸坯表面测温试验对模型的准确性进行了验证.通过模型研究了过热度、拉速和二冷比水量对铸坯中心固相率以及凝固坯壳分布的影响,并结合高碳耐磨球钢BU的高温拉伸试验结果,确定了最佳的拉速以及最优轻压下压下区间要求.通过工业试验对理论模型进行了验证,并分析研究了拉速对采用凝固末端电磁搅拌(F-EMS)以及凝固末端17 mm大压下量的轻压下技术生产310 mm×360 mm断面大方坯高碳耐磨球钢BU铸坯的偏析和中心缩孔的影响.结果表明:采用凝固末端电磁搅拌和轻压下复合技术,通过调整拉速优先满足轻压下压下区间要求,可显著降低中心偏析、V型偏析及中心缩孔,但如果仅达到凝固末端电磁搅拌位置要求时,则铸坯中心质量不会得到明显改善.拉速为0.52 m·min-1且轻压下压下区间铸坯中心固相率为0.30~0.75时,偏析和中心缩孔有很大程度的改善,不合理的压下量分配会引起铸坯出现内裂纹以及中心负偏析.      

45钢大方坯凝固传热数值分析

以某钢厂45钢大方坯为研究对象,建立了45钢凝固传热的数学模型。用双混合模型计算了45钢的热物性参数,并通过射钉试验及坯壳测温测定窄面坯壳厚度和表面温度对模型进行了验证,结果表明该数学模型能够较好的模拟45钢凝固传热过程。模拟结果表明:在现行拉速0.5 m/min,过热度为35℃工艺条件下,合适的轻压下位置在距弯月面21.8~23.1 m处;铸坯出结晶器表面温度回升幅度大,容易导致表面裂纹;稳定拉速是提高铸坯质量的关键。改进后,中心疏松等级从1.5级降到不大于1.0级,中心缩孔不大于0.5级,最大碳偏析比小于1.12,铸坯表面裂纹率由3.25%降低到0.5%;铸坯凝固末端的凝固加速是由于铸坯中心大过冷条件下钢液中等轴晶的快速生长造成的。     

不同角部形状特厚矩形坯凝固传热及应变数值模拟

 为了更加有效控制和减少连铸坯的角部横裂纹质量缺陷,根据其形成的机制,针对两种新型铸坯模型,即圆角和倒角模型进行研究。通过建立特厚连铸矩形坯在凝固过程的传热模型并进行数值模拟,得到铸坯在凝固过程沿拉速方向上温度场和坯壳厚度的分布规律,并在此基础上建立热力耦合模型,分析铸坯的应力变化,讨论了产生裂纹的可能性。研究结果表明,通过对比传统直角模型,得出圆角和倒角模型对铸坯角部温度场和应力场两个方面的分布状况都有改善,即新铸坯模型角部温度在连铸矫直段有效避开了钢的高温脆性区,同时降低了铸坯角部的应力值,减小了角部裂纹产生的可能性。     

连铸轻压下过程的热力分析

以D32船板钢250 mm×2 400 mm连铸坯为研究对象,借助ANSYS数值模拟软件建立铸坯在轻压下过程中的热力耦合模型,模拟分析了在轻压下过程中铸坯的变形特点和应力分布规律,为深入解决轻压下过程中铸坯芯部未完全凝固区的温度、相变、收缩等状态较强的指导意义。研究表明,二冷第9段至第11段辊缝逐渐收缩,铸坯厚度方向累积压下量逐渐增大,达到压下的目的;夹辊辊缝的持续缩小,使得铸坯芯部未凝固区被持续挤压减小,铸坯内部未凝固(液芯)部分主要来承担压下辊对铸坯所实施的作用;在轻压下实施过程中铸坯横断面各位置等效应力大小分布为角部宽面窄面。     

压下对重轨钢大方坯内裂纹敏感性的影响

为研究压下对连铸坯内部裂纹产生的影响,利用ABAQUS有限元软件建立了230 mm×280 mm断面大方坯压下数学模型。通过压下模型对重轨钢连铸坯压下过程进行热力耦合模拟计算,对压下过程中产生的内部裂纹进行了预测。首先,对连铸坯不同中心固相率为0.3～0.7的温度场进行计算;然后,利用压下模型计算了连铸坯中心固相率0.3～0.7时凝固前沿的等效塑性应变。研究结果表明,在连铸坯中心固相率为0.3～0.7的位置处分别施加7 mm压下量进行压下,连铸坯凝固前沿等效塑性应变未超过临界等效塑性应变（0.4%）,连铸坯未出现内裂纹;同时,对连铸坯在中心固相率为0.6位置处进行了不同压下量的研究,研究结果表明,当连铸坯压下量超过7 mm时,凝固前沿的等效塑性应变超过临界塑性应变（0.4%）,连铸坯出现内裂纹,并且压下量越大,连铸坯内裂纹越严重。同时,工业试验结果与模型计算结果基本吻合,验证了模型计算的准确性。      

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 on Initial Defects Based on Mechanical State of Meniscus Shell

The meniscus shell plays an important role in slab quality and process operation for continuously cast steel.One decisive reason is initial solidifying shell and growing dendrite under the mechanical stress caused by mold oscillation and liquid steel flow to generate disturbance of casting.The mechanical state of meniscus shell was analyzed using mathematical models in combination with thermo-physical properties and flow rate of steel to shed light on the formation of initial defects.The results show that the mold oscillation is a critical factor on the initial crack formation because the periodic stress makes the shell bending.The formed crack may also expand and propagate due to the following secondary cooling and straightening behavior.The primary dendrite has high possibility to be broken by fluid flow in the solidification front to lead to the non-uniform thickness of solidifying shell.The inter-dendrite bridging is also likely to be formed to produce other internal defects,such as air hole and solute enrichment in the residual molten steel located in the bridging area.     

On the Formation of Interdendritic Internal Cracks During Dendritic Solidification of Continuously Cast Steel Slabs

In this study of solidification during the continuous casting of steel slabs, the effect of the different spray cooling conditions on the interdendritic internal cracks formed between the columnar dendrites has been examined by a metallographic study of the slab samples and by performing a set of mathematical analyses. The metallographic study involved plant trials to measure the slab surface temperature within different secondary spray cooling conditions. Also, macro/microexaminations of the collected samples from plant trials, measurements of dendrite arm spacing, and interdendritic distance between the columnar dendrites, as well as a segment length of interdendritic crack, have been performed. The experimental results show that the morphology of the interdendritic cracks described by the segment width and length fluctuate with the distance from the slab surface based on the secondary spray cooling conditions. A one-dimensional mathematical model of the heat transfer, solidification, structure evolution, interdendritic strain, and elementary interdendritic area (EIA) has been developed. This model takes into account also calculating the width of interdendritic crack. The model predictions are in a good agreement with the measurements. The results pointed out also that this criterion can be considered as the most important tool to measure the inner quality of the continuously cast steel slabs. Therefore, it helps also to define the required mechanism and reduction level of hot working deformation to close these interdendritic internal cracks. The formation mechanism of these cracks during the dendritic solidification of continuously cast steel slabs has been discussed and the available solutions have been proposed.     

1650板坯压下过程热/力学行为及压下工艺优化

为了控制梅钢1 650板坯连铸包晶钢过程铸坯内裂纹发生,基于梅钢1 650板坯连铸机生产实际,建立了1 560mm×230mm断面包晶钢铸坯凝固过程三维热/力耦合有限元模型,揭示了铸坯凝固过程各冷却区内的温度场分布规律和铸坯压下过程应力与变形行为演变规律。结果表明,铸坯在结晶器及零段内冷却强度大,沿拉坯及其垂直方向的温度分布梯度大;在实施铸坯凝固末端压下过程中,铸坯宽面中心与宽向1/4处的表面变形及应力变化较为同步,且靠近铸坯内弧侧凝固前沿的塑性应变最大,铸坯应力最大值集中在角部区域;目前梅钢包晶钢连铸压下区间设置不当,易引发铸坯产生内部裂纹。     

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.     

Deformation Characteristics of Steel Billets with Liquid Core

To investigate the deformation characteristics of billets with liquid core during soft reduction and to clarify the correlation between internal cracks and deformation of the billet in the mushy zone, a fully coupled thermo‐mechanical Finite Element Model was developed in ABAQUS, furthermore, casting and soft reduction tests were carried out in a laboratory strand casting machine. During soft reduction the temperature distribution, the stress and strain states in the billet were calculated, the deformation characteristics of the billet during soft reduction were determined and the relation between internal cracks and equivalent plastic strain as well as maximal principal stress was analysed. The results show that tensile stresses can develop in the mushy zone during soft reduction and the equivalent strain nearby the Zero Ductility Temperature (ZDT) increases with a decreasing solid fraction. Internal cracks can be initiated when the accumulated strain exceeds the critical strain and /or the applied tensile stress exceeds the critical fracture stress during solidification. In addition, the factors (reduction efficiency and internal cracks) that should be considered to determine the optimal parameter for the soft reduction were established.     

Crystallographic fatigue crack growth in incompatible Aluminum Bicrystals: Its Dependence on Secondary Slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in\([12\bar 1]\) tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystal- lographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems.     

Propagation of surface cracks on beam blank in the mould during continuous casting

ABSTRACT

We focus on crack propagation to investigate surface cracks in the mould during continuous casting, based on the crack initiation mechanism proposed in previous studies. The temperature and stress data of a solidified shell were extracted, and an extended finite element model based on the continuous damage theory of elastic–plastic materials was developed to simulate surface crack propagation. The results showed that, in the cracked area, stress concentration occurred at the crack tip, and the element split open and the crack propagated when the maximum principal stress in the stress concentration area reached the critical value. Prefabricated cracks in the fillet and web mainly developed into longitudinal cracks in the mould. The theoretical mechanism of this study was found to be the same as the crack propagation mechanism observed during the actual production of beam blanks. Thus, this study reveals the theoretical principle of crack initiation and propagation and can provide theoretical guidance for controlling surface cracks during beam blank continuous casting.     

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.