Study On Longitudinal Cracks Of Medium Carbon Alloy Steel Plate

With the development of economy, the steel is playing a more and more important role in architecture, shipbuilding, oil and other industries. But some batches of the steel plates appeared some longitudinal cracks, which increases the rate of unqualified products, during the process of production. Steel plate crack is one of the most difficult problems in the process of production. Small cracks could be eliminated by grinding method in industry. But the increase of the unqualified products can increase the grinding workload which affect the production efficiency, increase the economic costs and reduce the quality of the final product. A steel factory focused on the medium carbon alloy steel plate producing, but surface small cracks on the plate is a serious problem. In recent years, the extensive research has been carrying on to control the longitudinal crack, and they has made some progress. But they have not reduced the rate of surface cracks fundamentally. The longitudinal cracks occurred on the medium carbon alloy steel plate,combined with technological lines and production processes, the reason of the steel cracks, influence factors and the control measures were studied by different methods. The main results were as follows:The steel defects of Q345 steel were counted up and compared. It shows that the major of defects were small surface longitudinal cracks. The cross-section of longitudinal cracks on Q345 steel plates were investigated by metallurgical microscope,scanning electron microscopy(SEM) and energy disperse spectrum(EDS). The results show that a large number of oxide spots which caused by internal oxidation are near the crack. These oxide spots have been formed before the rolling process, while the slab in the process of heating and heat holding in furnace. The decarbonization is occurred in varying degrees near the cracks,which indicates that the cracks has been formed before the slab entered the furnace. Internal oxidation and decarbonization are occurred after the slab enter the furnace, and the oxide spots and decarbonization are formed after a while.In order to determine the crack was formed in the slab stage, the Q345 slab was analyzed by metallurgical microscope,scanning electron microscopy(SEM) and energy disperse spectrum(EDS).The results show that the chilled layer thickness of Q345 slab is thin. There is only part of the area exist chilled layer, and the distribution is extremely uneven. The Widmanstatten structure is serious overall the samples. This structure can reduce the plasticity of the steel,and it can cause crakes in the process of straightening. In addition,there are small cracks in the slab which can be further propagate in the subsequent process. All in all, the cracks are produced in slab stage.In order to improve the slab structure and reduce the cracks, the crystallizer water flow was optimized. Through the research of the macrostructure and microstructure of slab,it shows that changing the crystallizer water flow cannot promote the formation of chilled layer. Near the surface of Q345 slab has no chilled layer under conditions of three kinds of crystallizer water flow as 6100 L/min, 6800 L/min, 7500 L/min. Widmanstaten level has an increasing trend with the increasing of crystallizer water flow, and the lowest level at 6100 L/min and the microstructure is the most homogeneous. The results indicate that problems still existed in crystallizer cooling, and changing the crystallizer water flow cannot improve the crystallizer heat transfer.In order to improve the heat transfer of crystallization and make the slab structure better fundamentally. The physical and chemical properties of mold fluxes were improved by adopting new mold fluxes. By changing the mold fluxes, it shows that the slab which use new mold fluxes has small and homogeneous structure within 5 mm near the inner arc and the level of surface Widmanstaten significantly low. This structure can reduce the cracks. By means of replacing new mold fluxes and rolling the slab,and doing statistical analysis on the rate of cracks on the rolling steel plates,it shows that the longitudinal cracks on the steel plates are significantly reduced.