Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.     

工艺参数对高碳钢小方坯中心缺陷的影响

系统地研究了首钢三炼钢4号铸机拉速和比水量对优质高碳钢小方坯中心缺陷的影响。首先在典型工况下分别调整了拉速和比水量,将拉速2．6m／min降低到了2．0m／min,铸坯的中心偏析、疏松得到了改善,同时中心缩孔在一定程度上有所恶化,因此将拉速优化为2．2m／min。随着比水量的降低(1．0～0．7)L／kg,中心偏析先恶化,然后改善,中心疏松和中心缩孔均得到了改善,因此将比水量控制在0．7L／kg。最后将拉速和比水量进行了优化匹配。结果表明对于首钢三炼钢4号铸机将拉速控制在2．2m／min、冷却强度控制在0．8L／kg有益于优质高碳钢小方坯中心缺陷的改善。     

连铸坯枝晶组织参数的测定及其应用

介绍了连铸坯凝固的枝晶组织特征、形成机制及定量评定方法。通过对硅钢连铸坯凝固组织比例系数和枝晶间距的测定以及连铸坯低倍组织缺陷的统计分析,确定了凝固组织形态与铸坯成分的关系。分析结果表明,在一定限度内,随Si含量增加,铸坯的一次晶间距加宽,各类低倍组织缺陷大大减少。     

二冷喷淋强度对连铸坯凝固组织的影响

 推导了连铸过程中二冷喷淋强度与连铸参数之间的关系,比较了喷淋强度对2种断面连铸坯凝固组织的影响。结果表明,喷淋强度能更精确地反映铸坯凝固组织与冷却参数间的关系;铸坯表面的枝晶间距为20 μm左右,而中心区为200～350 μm;距表面相同距离,对角线区的枝晶比横向细小。喷淋强度相近时铸坯的凝固规律为：200 mm×200 mm断面中心区组织较细小,150 mm×150 mm断面近中心区组织较细小。从铸坯表面到中心整体上符合凝固曲线左侧规律,但断面大于200 mm×200 mm的连铸坯,中心和近中心区凝固时符合凝固曲线右侧规律。     

Characteristic Parameters for Dendritic Microstructure of Solidification During Slab Continuous Casting

 The operating conditions during the continuous casting process have a great effect on the microstructure of slab solidification, including primary and secondary dendritic arm spacing. On the basis of the analysis of available work, a revised expression for describing secondary dendritic arm spacing during solidification of slab continuous casting was presented, and the relation between the ratio of primary dendritic arm spacing to secondary dendritic arm spacing and the cooling rate was obtained.     

Influence of Process Parameters on Microstructure and Mechanical Properties of Friction-Stir-Processed Mg-Gd-Y-Zr Casting

Mg-10Gd-3Y-0.5Zr (wt pct) casting was subjected to friction stir processing (FSP) at a constant rotation rate of 800 rpm and varied travel speeds of 25, 50, and 100 mm/minute. FSP resulted in the generation of fine-grained microstructure and fundamental dissolution of coarse Mg₅(Gd,Y) phase at the grain boundaries, thereby enhancing the tensile properties significantly at both room and elevated temperatures. The grain size of the FSP samples decreased with the increasing travel speed, whereas the microstructure heterogeneity with the banded structure (onion rings) became evident at a higher travel speed. Tensile elongation of the FSP samples increased as the travel speed increased, whereas the highest strengths were obtained at the medium travel speed of 50 mm/minute. Higher strengths and greater elongations were observed for the FSP samples in the transverse direction (TD) than in the longitudinal direction (LD). After post-FSP aging, the strengths of the FSP samples were increased significantly with the TD and LD exhibiting the same strengths; however, the elongation was decreased remarkably with the TD having higher elongation than the LD. A variation of the tensile properties was discussed in detail based on the microstructure heterogeneity and fracture surfaces.     

逆相变退火对连铸坯组织和力学性能的影响

 研究了逆相变退火温度对0.1C5Mn钢连铸坯的组织结构和力学性能的影响规律,采用SEM进行组织结构的表征,利用XRD技术分析连铸坯退火后奥氏体含量,并测试了退火试样的力学拉伸性能。试验结果表明,连铸坯退火过程中发生奥氏体逆转变且在较低退火温度下有少量碳化物析出,随着退火温度升高,奥氏体含量先增加后减少,析出物逐渐溶解消失。提高退火温度可以显著提高试验钢的抗拉强度但却降低它的屈服强度,另外随退火温度升高,断后伸长率和强塑积先增高后降低。在625~650℃退火,可以获得20%~25%的伸长率。研究结果说明利用逆转变退火可以大幅度提高中锰钢铸坯的力学性能。     

Three-Dimensional CFD Simulation Coupled with Thermal Contraction in Direct-Chill Casting of A390 Aluminum Alloy Hollow Billet

Metallurgical and Materials Transactions B - A three-dimensional CFD model coupled with melt flow, heat transfer, and thermal contraction was developed to simulate the direct-chill (DC) casting...     

热变形参数对7050铝合金微观组织的影响

在Gleeble-1500热模拟实验机上对7050铝合金试样在变形温度为250℃～450℃,应变速率为0．01～10s^-1,变形程度为50％、60％和70％的条件下进行等温压缩实验。利用透射电镜（TEM）分析了合金在不同变形参数条件下的微观组织特征。     

小方坯连铸机角裂漏钢分析

对角裂漏钢机理及现场角裂漏钢事故进行统计分析,认为造成角裂漏钢的主要原因是铜管内腔的工艺参数及使用过程的角部磨损造成的出结晶器坯壳不均匀和二冷系统冷却的不均匀性。为此,采取优化铜管内腔的工艺参数、提高二冷系统的均匀性等措施。使角裂漏钢发生率由1．2％降到0．4％以下。     

工艺参数对压铸AM60B合金组织缺陷的影响

采用液态压铸技术,研究了压铸工艺参数对AM60B合金组织缺陷的影响。试验结果表明,当浇注温度为680℃、模具温度为180℃、压射速度3．0m／s,压射比压为75MPa时,压铸镁合金AM60B可以获得组织均匀细小、表面光滑、缺陷极少的铸件。     

16MnNb钢圆坯连铸二冷制度的研究

 以某钢厂圆坯连铸机为研究对象,建立了连铸坯凝固传热模型。在不同拉速下对280 mm断面圆坯二次冷却过程进行仿真优化,确定了16MnNb钢合适的二冷制度。根据仿真结果,在最小工作拉速(0.9 m/min)下,矫直点处铸坯内弧表面中心温度为947 ℃,有效避开了铸坯的二次低延性区。在最大工作拉速(1.2 m/min)下,铸坯出结晶器时,其凝固坯壳厚度为19 mm,二冷初期产生漏钢等质量问题的可能性较小。不同拉速下,横断面温度场分布均匀。经低倍检测发现,铸坯表面及内部质量良好,无裂纹、疏松、缩孔等质量缺陷。     

Evolution of Precipitate Microstructure During Creep of an AA7449 T7651 Aluminum Alloy

Creep forming is a process where plastic deformation is applied at the material’s aging temperature. It enables to obtain parts of complex shape with reduced internal stresses and finds applications, for instance, in the aerospace industry. In this article, we report in-situ small-angle X-ray scattering measurements during creep experiments carried out on an AA7449 Al-Zn-Mg-Cu alloy in the T7651 temper. In the range of temperatures of 413 K to 453 K (140 °C to 180 °C), we show that the initial microstructure is not stable with respect to the applied stress/strain. Accelerated precipitation coarsening is shown to occur, clearly related to the plastic deformation. This strain-induced microstructure evolution is shown to happen even at temperatures well below the aging temperature that has led to the initial temper.     

结晶器振动参数对连铸坯表面质量的影响

针对电炉钢连铸 130mm× 130mm、15 0mm× 15 0mm、15 0mm× 2 10mm方坯表面出现的振疤、振沟对轧材表面质量的影响 ,在现有正弦振动模式下结合工业性试验对影响铸坯表面质量的负滑脱率、负滑脱时间、振动频率、结晶器导前等结晶器振动参数进行优化。实验表明 ,随负滑脱率 (负滑脱时间与振动周期之比 )降低 ,铸坯表面趋于平整 ,消除了沟形振痕。 15 0mm× 15 0mm方坯在拉速 2 0～ 2 8m min ,负滑脱率 33% ,振频 130～ 180 min ,负滑脱时间 0 13～ 0 16s ,振程设置 9 6～ 10 2mm时 ,明显改善了铸坯表面质量     

Influence of Billet Size on Flow, Solidification and Solute Transport in Continuous Casting

ListofSymbol　　B———Buoyancy ,m·s- 2 ;　　c———Concentrationofsoluteelement ;　　Cμ———Turbulentconstant;　　D———Diffusivityofsoluteelement ,m2 ·s- 1 ;　　fl,fs———Liquidandsolidfraction ;　　fμ———Turbulentcoefficient ;　　h———Enthalpy ,J·kg- 1 ;　　k———Turbulentkineticenergy ,m2 ·s- 2 ;　　kp———Equilibriumpartitioncoefficient;　　Kp———Permeabilityofmushyzone ,m2 ;　　K0 ———Permeabilitycoefficient;　　p———Pressure ,Pa ;　　Pr———Prandtlnumber ;…     

小方坯保护浇注及对铸坯质量的影响

介绍了莱钢炼钢厂３连铸机小方坯保护浇注工艺以及对耐火材料的改进措施,实践证明,采用保护浇注工艺,铸坯检验合格提高,缺陷减少。     

连铸坯内部裂纹产生的主要原因及解决措施

以莱钢50 t电炉生产线及新二区转炉炼钢生产线生产的两种规格的连铸坯作为研究对象,分析了内部裂纹形成的原因,并采用扫描电镜和能谱分析了内部裂纹的类型。通过采取恒拉速浇注、拉坯速度与水量合理匹配、实行中间包窄温度波动控制、提高钢水纯净度等措施,连铸坯的质量得到了明显改善,低倍和热顶锻合格率也有了显著提高。