The rolling mill housing is an important load-bearing part in the wide and thick plate rolling mill, and the rolling mill stands are required to be resistant to high temperature, vibration, gravity and impact loads. Its product structure is complex, with large wall thickness and size specifications, which puts strict requirements on the casting quality of the rolling mill frame.
 
Tenfaye has accumulated rich experience in rolling mill housing technology and production. By using MAGMA software to perform numerical simulation of the solidification process of the rolling mill frame, it can effectively predict the areas in the castings where shrinkage, looseness and cracks may occur, and take targeted process measures to achieve process optimization and quality improvement of large rolling mill stands castings, and reduce production costs.
 
1　Technical requirements for rolling mill housings
Figure 1 shows the wide and thick plate rolling mill frame casting produced by the company, with a net weight of 195 t, a main wall thickness of 760 mm, and a maximum profile size of 11 415 mm×5 085 mm×1 790 mm. The weight and size are large, the frame has many non-processed surface areas, the dimensional accuracy control is difficult, and the overall quality requirements are high. 1.1　Chemical composition and mechanical property requirements
The casting material is G20Mn5, and the heat treatment state at the time of delivery is normalizing + tempering. The size of the attached cast test block is 250 mm×200 mm×150 mm. The test block size is large, and the test block quality requirements are high. The chemical composition and mechanical property requirements comply with the provisions of EN 10293:2015, as shown in Table 1 and Table 2.  
1.2　Nondestructive testing requirements The nondestructive testing process has high requirements for the overall external and internal quality of the frame. 100% MT inspection is carried out in accordance with EN 1369, with acceptance levels of SM2 and LM/AM3, and acceptance levels of SM2 and LM/AM2 for weld repair areas; 100% UT inspection is carried out in accordance with EN12680-1, and the acceptance levels are implemented in accordance with Table 3. The acceptance requirements not specified in Table 3 (100 mm < wall thickness ≤ 600 mm) are accepted in accordance with Level 4 of EN12680-1, and are applicable to areas with wall thickness ≥ 600 mm. During UT inspection, K1 or K2 dual crystal probe inspection is added to the 100 mm special edge area of the frame window (including the window fillet) to reduce weld repairs caused by surface defects after finishing of the key area of the frame window. 2 Casting process analysis
The overall structure of the frame is complex, and the molding operation is difficult. The boss fillet area is prone to cracks due to the shrinkage of the casting. The cross-section is thick, the heat node is large, the modulus is large, and the overall structure is rod-shaped. The riser shrinkage compensation distance is long, which is prone to shrinkage defects and coarse grains after heat treatment. The total weight of the molten steel is 390 tons, and the sand mold rigidity and strength are high to avoid the risk of shrinkage, bending and cracks of the casting caused by the movement of the mold wall. The material is low-carbon low-alloy steel, the pouring time is long, the molten steel has poor fluidity, and the solidification method tends to be paste solidification, which is also prone to shrinkage defects. The performance of the test block is easily unqualified due to component segregation [3,4]; the quality requirements of the special area of the window are high. It is necessary to ensure the quality of the inside and near the surface of the column, and also consider the influence of the sand mold yield on the mechanical shrinkage of the casting; the non-destructive testing operation has high requirements for surface roughness, and it is necessary to improve the quality of the non-processed surface and dimensional accuracy control by setting process correction, grinding, tool correction, etc.
 
3 Casting process design and numerical simulation
3.1 Casting process plan
3.1.1 Selection of parting surface
The parting surface should be selected so that all or most of the casting is placed in the same half mold. A flat surface should be selected as much as possible and the number of parting surfaces should be reduced. At the same time, the influence of the process plan on the molding operation should be considered. According to the frame structure and the distribution of the bottom surface cold iron, the plane of the screw hole pressing down end is used as the casting bottom surface, and the upper and lower end surfaces of the column are used as the parting surface. Most of the bosses are placed in the upper mold to minimize the influence of the lower mold boss on the shrinkage of the casting and improve the convenience of the lower mold operation. At the same time, the boss under the riser can be fully utilized to enhance the shrinkage compensation effect on the casting body and improve the casting quality. The parting surface selection plan is shown in Figure 1a.
 
3.1.2 Shrinkage compensation system design
The dimensions of the press-down end, the foot end crossbeam and the column riser are determined by the hot pitch circle method, and the shrinkage compensation capacity of the riser is verified by the modulus method. The L compensation distance of the frame column is ≈9.5T hot section. The column of the rod-shaped structure is prone to axial looseness defects in the center. Therefore, two risers are set on each column to increase the continuity of the riser and shorten the compensation distance of the column riser. According to the distribution of chillers on the bottom surface of the column and the calculation results of the compensation distance of the beam and column risers, the frame adopts a process scheme of six waist-round risers to concentrate the shrinkage of the thick parts of the casting to improve the shrinkage compensation effect of the riser. The riser process scheme is shown in Figure 2.
 
According to the wall thickness of the main body of the frame, a special dark chill is set between the risers to form an artificial end zone. The dark chill is arranged in a step-like manner to form a temperature gradient pointing to the riser, increase the expansion angle of the shrinkage compensation channel, extend the riser action area, and improve the casting quality of the special area of ​​the window; setting a special dark chill at the boss and the window fillet can effectively avoid the generation of thermal cracks; setting a dark chill at the bottom of the pressing end, and setting an air outlet channel at the screw hole sand core to increase the cooling speed of the lower part of the nut hole and avoid the risk of cracks in the nut hole caused by thermal stress.
 
3.1.3 Design of pouring system
A double-layer open stepped pouring system is set at the bottom and riser of the frame. The cross-sectional area ratio of each unit of the lower pouring system is S ladle: S vertical: S horizontal: S internal = 1: 1.96: 2.35: 3.33, and the cross-sectional area ratio of each unit of the upper pouring system is S ladle: S vertical: S horizontal: S internal = 1: 1.96: 3.92: 5.12, to ensure that the molten steel fills the mold smoothly and realizes the temperature gradient and sequential solidification from the bottom to the top; according to the company's experience in producing frames, the inner gate is placed 15~20 mm away from the frame body, and the brick pipe spacing is controlled to be more than 300 mm, which can effectively avoid cracks and loose defects at the physical hot node near the gate.
 
The frame has a large cross-sectional size, adopts the cavity argon blowing protection pouring method, and follows the principle of low temperature and fast pouring, which can reduce the inclusion defects caused by the secondary oxidation of the molten steel during the pouring process. At the same time, the pouring temperature is controlled at 1 530~1 550 ℃ to reduce the superheat of the molten steel, reduce the influence of component segregation, coarse grain risk and shrinkage defects on the quality of castings during the solidification process.
 
The influence of the pouring process on the quality of castings is crucial, which is not only reflected in whether the pouring system design is reasonable, the pouring scheme (composition, temperature control), etc., but also in the production process. Attention should be paid to factors that affect the quality of large steel castings, such as molding raw and auxiliary materials, heat treatment process and casting cleaning.
 
3.2 Numerical simulation of casting process
Using MAGMA software and combining the Feeding module based on the Niyama criterion method, according to the UT detection requirements of the rack, the Niyama function criterion value is selected as 0.8 ℃1/2·min1/2·cm-1, and the numerical simulation results of the casting process are shown in Figure 2. The solidification simulation results show that there are no shrinkage defects inside the casting, and the final solidification area is concentrated in the riser. The pouring height and shrinkage feeding capacity of the process riser can meet the quality requirements of the rack. The temperature field change during the solidification process is shown in Figure 3. The dark chill set between the risers enhances the end cooling effect during the solidification process of the casting, realizes the zoned shrinkage compensation effect of the risers, and effectively eliminates the axial looseness defects that may occur between the risers. The dark chill with rounded corners of the window shows a good cooling effect in the early stage of solidification, accelerates the solidification and cooling rate of the tissue in this area, and effectively avoids the risk of stress-induced cracks. The modulus of the beam area is large, the solidification time is long, and there is a large simultaneous solidification zone inside. The quality of this part of the casting still requires a reasonable heat treatment process to ensure. The change law of the solidification temperature field shows that the casting as a whole has achieved sequential solidification from bottom to top, indicating that the riser and chill design in the process plan is reasonable and can be used in the actual production of the frame. 4 Production inspection
The production of the large-scale wide and thick plate rolling mill housing has been completed using the designed process plan. The performance test of the test block after heat treatment is qualified, and the mechanical property test results are shown in Table 4. There is no coarse grain in the internal structure of the casting under the riser. The non-destructive testing process is passed once by the user's joint inspection. The MT and UT test results meet the standard requirements. The overall quality of the frame is good and recognized by the user.
 
The rolling mill stand has large dimensions and complex structure. First of all, the parting surface should be reasonably selected. MAGMA software can verify the rationality of the chill, riser and pouring system in the frame process plan. The quality inspection results of the actual produced frames meet the standard requirements.