| 其他摘要 | The railway frog is one of the most important parts that change the way of train. The frog point is subjected to large impact force and shearing force. The fabrication process of an alloyed steel frog point is complicate and of high cost. Once a crack forms in the frog, it propagates quickly. As a result, the frog must be replaced in time for security reason. Nowadays, the alloyed steel frogs can not meet the railway requirements, such as high speed, high security and heavy-duty. The high manganese steel frog point shows noticeable advantages over the alloyed steel one, such as easy casting, good performance and low crack propagation rate, which guarantee the safety of a train. However, the frog point is a heavy-section casting; and there exist many defects in the traditional castings, such as oxides, microcracks and coarse grains. These defects limit the application of the high manganese steel frog point. In this paper, a new and smooth filling system is developed based on the computer simulation and X-ray real-time observation, which improves the purity of the steel melt. The grain size of a high manganese steel casting is refined obviously by a micro-alloying technique and the addition of metal particles technique, and the mechanical properties are improved. The main results are summarized as followings:
(1) A new and smooth filling system is developed by combining the computer simulation and X-ray real-time observation. Based on this system, a smooth filling process is obtained, the air entrapment, turbulence and inclusions are avoided. By this way, the filling system of TiAl exhaust valves is also improved by changing the ingate size and order. The castings benefit from sequential solidification as the feeding paths are kept open, which eliminates the porosities and increases the yield.
(2) Based on the numerical simulation, the mould filling and the temperature field of the frog point are calculated. According to the simulation results, cast defects such as gas entrapment, cold shut and porosities are predicted. The risers are optimized and the casting process parameters such as pouring temperature are defined, and the optimized casting process is presented.
(3) The mechanism of crack formation in the frog point is investigated in detail. An integrated filling/solidification/cooling/rises cutting/slow-cooling/thermal stress model is established, and the change of the thermal stress in high manganese steel frog point is predicted. The simulated results are in good agreement with the experiment results. It is indicated that oxides, carbides and phosphide eutectic weaken the grain-boundary strength and depress the casting performance, and act as crack sources under stress. It is concluded that the oxides and phosphorus must be controlled in order to eliminate the cracks.
(4) The mechanism of micro-alloying technique is investigated. The grains of the heavy-section and high manganese steel frog point are refined by a microalloying technique. It is found that the addition of small amount of vanadium and titanium can significantly reduce the grain size by two grades. As a result, the microstructure homogenization and the mechanical properties are improved. It is also observed that MnS inclusions are also refined and uniformly distribute in the castings.
(5) Another refinement technique, metal particles addition technique, is also developed. It is shown that addition of ting metal particles into the melt during the casting process increase the nucleation site density refines grains and improves the casting performance. The grain size is improved by 2-3 grades and the mechanical properties are also enhanced The evolution of the grain structure of the frog point during solidification processes is simulated based on cellular automation method. The solidification microstructures of high manganese steel heavy-section casting are refined by microalloying technique and metal particles addition technique. A batch of high manganese steel frog points with refined grain, homogenized microstructures, and excellent performance have been produced successfully, and passed on line evaluation. |
修改评论