Production process of ultra-thin wall cast steel underframe

   1. Introduction to castings
Large ultra-thin wall cast steel chassis (see Figure 1 for structure) for 120t flat trains. Products with similar structures used to be welded structures. The main problem is that during the use, due to the long-term pressure of various workpieces, the upper flat steel plate is deformed and the weld position is cracked. In order to improve the service life of the product, the company requires integral casting. The material is ZG230-450, the contour size is 13000mm×3000mm×(250~600)mm (the casting height is changed from 250mm to 600mm), and the average wall thickness is 30mm. The net weight of the casting is 27.3t. The modeling method uses a resin sand pit shape. The pouring temperature is controlled at 1585 to 1595 °C.

2. Difficulties in production process
(1) Since the wall thickness of the casting is too thin, this piece is produced by furan resin sand molding. First of all, due to the influence of thermal stress and mechanical obstruction stress during solidification of the casting, serious cracks may occur and the product may be scrapped. Secondly, due to uneven force on the casting, the casting is severely deformed.
(2) The rounded positions of the castings may cause the castings to be scrapped due to the occurrence of serious sanding defects.
(3) During the casting process, the castings are too long and the wall thickness is too thin, resulting in serious cold insulation, insufficient pouring and the like.
(4) During the molding process, due to the large number of sand cores, the wall thickness of the castings is thin, and the manual operation of the plugs is difficult. During the pouring process, the castings may be scrapped due to the drift of the molten steel along the core seam. All sand cores in the inner cavity can only be exhausted downwards. If the lower exhaust gas is not smooth, it will cause a smoldering accident, which will cause the castings to be scrapped.
3. Process analysis and countermeasures
(1) Cracks in castings are mainly affected by stress. First, the wall thickness of the casting is not uniform. During the solidification process of the casting, the speed of each part is inconsistent, resulting in inconsistent shrinkage of each part, resulting in internal stress. In the rounded transition position, the hot section is relatively large and the cooling is slow, and the position is mainly affected by the tensile stress; the connecting rib wall between the hot joints is thinner and the cooling is faster, and the position is subjected to compressive stress when the casting is Cracks occur when the internal stress exceeds the tensile strength of the metal itself. Secondly, the solidification shrinkage of the casting is larger than that of the sand core. In the solidification process, the sand core hinders the shrinkage of the casting, which may cause cracks in the casting.
The internal deformation caused by the contraction of various parts of the casting causes the overall deformation; the external force acts to deform the casting. The casting is deformed by the action of lifting force and friction when it is hitting the box.
In order to prevent crack defects, firstly, the inner cold iron is placed at each hot section, and the round steel bars of appropriate diameter are selected according to the "Code for the preparation of steel casting process" to achieve simultaneous solidification as much as possible, thereby ensuring uniform force of each part of the casting and reducing cracks. The tendency is to reduce the deformation tendency of the casting; secondly, it is required that all the sharp corners in contact with the molten steel in the cavity are smoothly transitioned, and the straw bundle is placed as a loose layer material at a distance of 100 mm from the surface of the casting at all corner positions, and the casting is reduced. It is subject to sand resistance when contracting.
Other requirements: In the process design, firstly, the pouring temperature and speed are strictly controlled to ensure the uniform temperature of the casting; when designing the lifting shaft, a plurality of hanging shafts are arranged at the symmetrical position to ensure uniform lifting force of each part of the casting. In terms of operation, in order to reduce the shrinkage resistance, the upper box pressure iron is removed about 30 minutes after pouring; in order to prevent cracks and deformation, the casting pit temperature is strictly controlled <300 °C; in order to prevent deformation due to lifting force, it is strictly prohibited to unilaterally rub the workpiece; Reduce the friction of the casting hoist and remove the molding sand around the casting and casting system before starting.
(2) Adhesive sand defects are mainly affected by many factors such as casting and molding conditions: 1 The pouring temperature is too high, and the surface of the liquid metal in contact with the air forms an oxide when pouring, which is easy to wet the silica sand mold. It is absorbed by the mold and further reacts with silica sand to form iron silicate, thus forming chemical sand. 2 The pouring speed is too fast, the scouring force of the molten steel turbulence is too large and the surface of the cavity is damaged to cause sand sticking. The strength of the type 3 sand is not enough or the core is not compacted, so that the compactness of the sand core itself is low, the gap between the sand particles is large, and the molten steel penetrates the surface sand and sticks sand. 4 The hot section is larger, because the surface wall is molten and sanded due to the long-term continuous high temperature.
In order to reduce the occurrence of sticking defects, it is first required from the casting conditions that the molten metal fills the cavity at an appropriate speed for a prescribed period of time. The casting temperature is required to be controlled at 1585~1595°C, and the liquid surface rising speed in the cavity is controlled at 10~12mm/s. Secondly, when the core making operation is required, the surface sand at each corner of the hot section is made of high refractoriness under high temperature conditions. The coefficient of thermal expansion is small, and the chromite ore resin with chemical stability is required, and the surface of the cavity is uniformly coated with the zircon sand powder alcohol-based paint, and the thickness of the coating layer is greater than 2 mm.
(3) Reduce the occurrence of defects such as cold insulation and insufficient pouring, ensure the formation of castings, improve the casting temperature of molten steel, and speed up the casting speed. Although the pouring temperature of molten steel is high and the pouring speed is fast, the tendency of sanding defects is increased, but at the same time, the filling ability of molten steel is increased, and the tendency of defects such as cold separation and insufficient pouring is reduced; secondly, before pouring is required Preheat the cavity. When the casting is molded with resin sand, the hot air is blown into the cavity for 3 to 4 hours, and the heating temperature is 150 to 180 °C, which not only helps to reduce the water vapor adhering to the surface of the cavity, but also helps to improve the casting surface. The type temperature increases the heat storage of the sand type itself, and reduces the heat loss after the contact between the molten steel and the wall, thereby increasing the filling ability of the molten steel.
(4) Preventing the core seam from drilling steel and causing a drifting core accident. According to the conventional process, if each individual cavity forms a sand core, the inner cavity core has a total of 70 pieces. More cores, more core seams, difficult operation and increased risk of steel and bonfire. When designing the process, the core is divided, breaking the convention, connecting several cores together (see Figure 2), and requiring an integral core box to reduce the core seam. In addition, when the hard sand bed is used, the surface sand is required to be doped with iron beans, the weight ratio of the molding sand to the iron beans is 4:1, and the thickness of the sand layer is about 40 mm, so that the molten steel of the drilling seam is rapidly solidified in the cold. In order to prevent the floating core, the process design requires that the solder joints be reserved at the core position of each core when the core is made, and the lower core is welded and fixed to be integrated, and a hard top column of φ12 mm is placed above each core before the box is assembled.

In order to ensure the smooth discharge of the lower exhaust, firstly, the exhaust system is placed in the hard sand bed, and five seamless steel pipes with a diameter of 120 mm and burrowed around the body are placed in parallel at intervals of about 600 mm, and the grass is placed appropriately. rope. All the exhaust cords or nylon ropes in the core are placed in a radial distribution, and the spacing between the layers is 200-250 mm. The joints of the layers form a bundle and are finally led out of the core.
4. Conclusion
The castings were successfully cast at one time (see Figure 3). There were no cracks or sticky sand defects on the surface and corners of the castings, and there were no defects such as cold insulation and insufficient pouring. The surface quality and actual size of the casting blanks met the requirements of the drawings. Practice has proved that the success of the preventive measures taken can provide reference and reference for the manufacturing process of large-scale thin-walled castings.

About the author: Zheng Xinqiu, Yao Jianzhu, Technical Quality Department of China's Heavy Casting and Forging Steel Division.

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