Q345钢连铸坯质量研究
[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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