Q345钢连铸坯质量研究

发布时间：2018-08-03 08:42

【摘要】：随着桥梁建造、公路运输的发展,Q345钢的应用也越来越广泛。这就要求Q345钢即使在严苛的服役环境下也能保持良好的性能,而铸坯往往伴随着表面横裂纹等缺陷,虽然横裂纹可以通过热吹扫的方法予以消除,但热吹扫工序费时费力,严重影响生产效率。目前,国内某钢厂生产的260mm×2070mm规格的Q345钢连铸坯存在严重的表面横裂纹,本文针对Q345钢表面横裂纹问题,从铸坯凝固行为入手,以调整连铸工艺参数为技术手段,深入分析铸坯表面横裂纹产生的原因、影响因素和控制措施,从根本上控制铸坯表面裂纹。针对以上问题,利用金相显微镜、扫描电子显微镜等对Q345钢连铸坯典型裂纹试样进行分析发现,裂纹主要分布在铸坯角部附近表面,并沿着晶界上的先共析铁素体薄膜向铸坯内部延伸,在显微镜下对先共析铁素体薄膜进行统计分析,发现其厚度偏大;同时在奥氏体晶界上有第二相质点析出,呈链状分布,脆化了晶界,在应力作用下易形成空穴,不利于控制裂纹,对裂纹内部取点进行成分分析,发现不含保护渣成分,推断裂纹应该是在铸坯出结晶器弯曲矫直的过程中受应力作用形成的。对铸坯全厚度试样进行低倍检验,发现其等轴晶率较低,柱状晶发达;铸坯窄面振痕深度较大,有可能导致铸坯窄面传热不均匀,易产生裂纹。通过金相显微镜观察铸坯微观组织发现,其表面为一定厚度的激冷层,内部为魏氏组织,对铸坯表面激冷层厚度进行统计分析,发现在铸坯窄面区域,激冷层厚度很薄或者不存在激冷层,内部的魏氏组织在应力作用下就容易导致开裂;先共析铁素体在铸坯窄面呈膜状分布的现象也比较明显,这些连续分布的软相也有利于裂纹的扩展;同时在铸坯外弧角部发现索氏体组织区域,很可能是由于结晶器锥度过大造成的,过大的结晶器锥度还会使铸坯所受摩擦力增大,在拉坯过程中出现表面横裂纹。综合以上研究结果,参考目前连铸结晶器锥度参数(1.1%-1.2%),制订了结晶器锥度分别为0.9%、1.0%和1.1%的三种不同工艺方案,对不同工艺参数下的铸坯质量进行研究,对比发现,当结晶器锥度为1.0%时,铸坯激冷层厚度最大,铸坯组织也相对更加均匀;用维氏硬度计进行测量,其硬度数据分布也相对集中,组织过渡比较均匀;说明针对Q345钢连铸坯,当结晶器锥度为1.0%时,能够较好地补偿坯壳凝固收缩带来的气隙,保证铸坯表面传热良好。最终研究结果表明,该厂连铸过程中原结晶器锥度参数设置偏大,导致铸坯出现表面横裂纹,工艺改进后,结晶器锥度参数设置为1.0%时铸坯质量明显改善。
[Abstract]:With the construction of bridge, the application of Q345 steel is more and more extensive. This requires that Q345 steel be able to maintain good performance even in severe service conditions, and the billet is often accompanied by defects such as transverse cracks on the surface. Although transverse cracks can be eliminated by means of hot sweep, the hot sweep process is time-consuming and laborious. The production efficiency is seriously affected. At present, there are serious surface transverse cracks in Q345 steel continuous casting billet of 260mm 脳 2070mm specification produced by a domestic steel factory. Aiming at the problem of surface transverse crack of Q345 steel, this paper starts with the solidification behavior of the billet and takes adjusting the technological parameters of continuous casting as the technical means. The causes, influencing factors and control measures of the transverse cracks on the billet surface are analyzed in depth to control the surface cracks of the billet fundamentally. In view of the above problems, the typical crack samples of Q345 continuous casting billet were analyzed by metallographic microscope and scanning electron microscope. It was found that the cracks mainly distributed on the surface near the corner of the billet. The proeutectoid ferrite film extends to the inner part of the slab along the grain boundary. The statistical analysis of the proeutectoid ferrite film under the microscope shows that the thickness of the proeutectoid ferrite film is too large, and at the same time, the second phase particle precipitates on the austenite grain boundary, which is distributed in chain shape. When the grain boundary is embrittlement, holes are easily formed under stress, which is not conducive to controlling the crack. The composition analysis of the point taken from the crack is carried out, and it is found that there is no composition of the mold powder. It is inferred that the crack should be formed by stress during the bending and straightening of the casting mould. It is found that the full thickness specimen has low equiaxed crystal ratio and developed columnar crystal, and that the depth of vibration marks on the narrow surface of the billet is large, which may lead to uneven heat transfer on the narrow surface of the billet and easily produce cracks. The microstructure of the billet was observed by metallographic microscope. It was found that the surface of the billet was a chilling layer with a certain thickness and the internal structure was Weiss's structure. The statistical analysis of the thickness of the quench layer on the surface of the billet was carried out. The thickness of the chilled layer is very thin or there is no chilling layer, the internal Weiss structure is easy to crack under the stress, and the phenomenon of preeutectoid ferrite distribution in the narrow surface of the billet is also obvious. The continuous distribution of soft phases is also beneficial to the crack propagation, and at the same time, it is found in the outer arc corner of the billet that the structure area of the Soxhlet body is probably caused by the excessive taper of the mould, and that too large the mold taper will increase the friction force of the billet. Transverse surface cracks appear in the process of billet drawing. Based on the above research results and referring to the taper parameters of continuous casting mould (1.1- 1.2%), three different technological schemes of mold taper of 0.9g 1.0% and 1.1% are worked out. The quality of billet under different technological parameters is studied and compared. When the mold taper is 1.0, the thickness of the quench layer is the largest and the structure of the billet is relatively more uniform. The hardness data distribution is relatively concentrated and the microstructure transition is more uniform when measured by Vickers hardness meter. When the mold taper is 1.0, the air gap caused by the solidification shrinkage of the shell can be compensated well, and the heat transfer on the surface of the billet can be ensured. The results show that the original mold taper parameter is too large during the continuous casting process, which leads to the appearance of surface transverse cracks. After the process is improved, the casting quality is obviously improved when the mold taper parameter is set to 1.0.
【学位授予单位】：郑州大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TF777

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