The top plate casting is an important part of the supporting transportation equipment for metallurgical vehicles. Its structure is shown in Figure 1. The material of the product is cast steel zg230-450, the unit weight is 7.1 T, and the maximum overall dimension is ϕ 2 900 mm × 450 mm, plate thickness 130 mm, rib thickness 60 mm. The internal quality of castings is high, and cracks, shrinkage cavities, porosity, air holes, sand holes, slag inclusion and cold shut are not allowed. Magnetic particle inspection shall be carried out on the working surface of the platform after machining, and no casting defects affecting the quality of the whole casting are allowed.
It is easy to produce shrinkage cavity and porosity in the center of large plane. How to eliminate shrinkage porosity is the key and difficult point of the process.
- Molding process
In order to facilitate the sequential solidification and feeding, the pouring position as shown in Fig. 2 is determined. The two box scraper molding process is adopted. The whole flange workpiece is placed in the lower box, and only the riser is left in the upper box. In this way, it is not only convenient to close the box, but also convenient to ensure the casting size.
- Gating system
The open gating system is adopted and the inner diameter of the sprue is selected ϕ 70 mm fire-resistant porcelain pipe, and the ingate is made of stainless steel ϕ The 40 mm refractory ceramic pipe enters along the bottom of the work piece corresponding to the riser, and the runner is protected by the refractory ceramic pipe, which effectively avoids the occurrence of sand flushing.
This gating system has the advantages of simple and reliable structure, easy cleaning, smooth continuous filling, suitable filling time, complete and clear casting contour, strengthening feeding effect of riser and reducing casting stress.
The inclined pouring process not only reduces the baking time of molten steel to large plane, but also reduces the possibility of sand inclusion, sand falling and other defects; At the same time, the high-temperature liquid steel in the high position can fully feed the low position liquid steel in the solidification process under the self weight pressure. In the process of gate through riser, the final heat source is at the riser, which ensures that the uppermost part of the casting has a smooth channel in the final solidification process, and there is sufficient high-temperature liquid steel to feed it, so as to avoid defects such as shrinkage porosity and cracks to the greatest extent.
- Riser design
As shown in Fig. 2, the insulating riser and inclined pouring process are adopted, and only one insulating open riser with the size of ϕ 550 mm × 800 mm。 The liquid steel near the edge of the riser crystallizes rapidly on the periphery, which makes the effective diameter of the riser smaller and forms a cold barrier on the contact surface of the second and third supplementary pouring of liquid steel. In order to eliminate this phenomenon, it is necessary to increase the riser diameter and riser height, but this will reduce the process yield. The size and quantity of risers are reduced, and the height of risers is reduced by at least one third.
According to the formula, the diameter of insulating riser d = 550 mm is obtained, so that the number of risers is reduced from the original 6 sand mold risers（ ϕ 280 mm × 380 mm) to the present one insulated open riser (combined with production experience, the size of insulated riser is selected as ϕ 550 mm × 800 mm）。
The feeding volume (V0) of the liquid metal supplied by the sand mold riser to the casting can reach up to 14% v sand (V sand is the volume of the sand mold riser). If the feeding volume (V0) of the same casting is the same, the feeding volume of the insulating riser is as follows:
V0=0.14 π( D sand) 2 / 4 × H
D sand = ed
D – insulation riser diameter;
D sand – riser diameter of common sand mold;
E – modulus increasing coefficient of insulating riser, generally e = 1.3 ~ 1.4.
Generally, riser height h = 1.2D, D is riser diameter.
For the castings with good feeding channel, the total volume shrinkage is 8%, that is, the weight of liquid metal to be fed is 0.08w (W is the weight of casting). So 0.08w parts = v0 ×γ（ Specific gravity of cast steel γ= 85), then d = 0.28 (W part) 1 / 3 = 0.28 (7.1) 1 / 3 = 538 mm (rounded to 550 mm).
After checking, the riser is sufficient for the amount of liquid steel needed for casting feeding, and the riser is insulated ϕ 300 mm × The size of 400 mm is reasonable.