Due to the lack of strict control of raw materials, unreasonable process plan, improper production operation, imperfect management system and other reasons, adamite rolls will produce such as pores, sticky sand, slag holes, residues, and shrinkage holes, shrinkage porosity, cracks, and hardness. Equal all kinds of quality defects.
1. Steel purity control technology
The roll molten steel purity control technology runs through all links such as batching, primary refining, refining, casting, etc., and adopts the full-process inclusion control technology to control endogenous inclusions and foreign inclusions, significantly reducing the number of inclusions, reducing large-sized inclusions and improving their quality. Distribution and morphology in steel. Using composite deoxidation of ferromanganese, silicon calcium, rare earth magnesium, etc., the oxygen content is reduced to (40-60) × 10-6, which is greatly reduced compared with the previous Al deoxidation.
2. Chemical composition control technology of molten steel
Adjust the content of carbon and alloying elements in adamite rolls according to different purposes, optimize the narrow composition control of the alloy, add V and Ti elements, and play the role of grain refinement and dispersion strengthening. Under the combined effect of dispersion strengthening and precipitation strengthening, The purpose of improving the microstructure and performance of the roll is achieved. The molten steel adopts the rare earth inoculation method to isolate and pelletize the carbides to increase the wear resistance. The existence of pearlite in the tissue in different forms will affect the formation of tissue stress and internal stress generated by heat treatment. Flake or needle-shaped pearlite may lead to the formation of micro-cracks, which may expand cracks under the promotion of internal stress. And finally cause the roll to crack, and after the formation of granular pearlite, the occurrence of hot cracking tendency is greatly reduced, and the accident resistance of the roll is also improved.
3. External quality control technology of roll
1) Shrinkage cavity, shrinkage (loose)
The roll product has a special structure, a large cross-section, and a long axial direction, which inevitably cause casting defects such as shrinkage and porosity during casting. Shrinkage cavities are likely to occur at the riser end of large semi-steel rolls, and shrinkage (sparse) porosity is likely to occur at the lower roll neck. The shrinkage cavities and porosity defects that appear are within the working face of the roll body and affect the quality of the roll. The casting process design adopts technological measures such as lower roll neck sand mold ≤30 mm, roll body cold mold hanging sand 12 mm ~ 15 mm, riser sand mold ≥ 60 mm and other technological measures; accurately control the pouring temperature, generally 50 ℃ ~ above the liquidus line 60℃; Strictly control the temperature of the cold mold, generally 80℃～100℃; the time of spot casting the riser with high temperature molten steel is generally controlled within 30 minutes, the temperature exceeds the liquidus line by 80℃～100℃, and the distance below the upper plane of the riser Start to pour the riser at 400 mm, the pouring time is more than 6 minutes, the filling is up, add 2 kg/t of exothermic agent, and cover the insulation cover; cite the self-developed patented electric heating technology, and adopt a reasonable electric heating process to make The feeding channel of the riser remains unblocked; the size of the riser is designed to meet the feeding requirements by using solidification simulation technology to ensure the molten steel required for volume shrinkage.
Taking the above process measures can slow down the generation of shrinkage cavities and shrinkage (porosity) defects.
2) Slag inclusion, sand inclusion
Use a special grinding wheel to polish the inner surface of the cold mold, remove the dirt on the inner cavity of the cold mold and other sandboxes, and ensure that the surface is free of adhesives. Sand molds are dried by slowly heating up to 400°C. Before closing the box, the joints should be ground flat, the inner cavity and pouring pipe debris should be sucked out, and the quality of the cavity should be strictly checked. If there is any damage, it should be repaired and dried before use. The tangential inner runner is used to make the impurities concentrated in the center of the roll, so as to facilitate the floating of the inclusions, and the sprue cup adopts a stable closed step design.
3) Cracks, broken rolls
There are hot cracks and cold cracks in casting cracks. The hot cracks are generally caused by the process. If the axial shrinkage is blocked, it is easy to cause hot cracks. The roll can shrink and deform freely, and it must have a good sequence of directional solidification from the bottom to the top; the model is as close to the shape of the final roll as possible, and the columnar crystal structure with a good surface can show good wear resistance in the final roll. The pouring process parameters must be strictly implemented, and the support body of the insert section must be evacuated in time. Elements such as Ni, Mn, and Mo increase the sensitivity of white spots and are prone to cracks.
The drying of raw materials into the furnace must be well controlled. Causes of Cracks in heat treatment: After the roll is subjected to high-temperature diffusion annealing, transverse cracks are generated at one-third of the roll body from the lower roll neck end, which occurs in the high-temperature diffusion annealing stage of heat treatment, and the temperature range is between 680 ° C and 1 040 ° C.
There are mainly two kinds of stress in the cooling and heating process of the roll, namely thermal stress and tissue stress. During the heating and heating stage of heat treatment, the surface is heated and expanded to produce tensile stress in the axial and radial directions, and compressive stress is generated in the place where the temperature is relatively low. From the position of the rolls and the temperature distribution in the furnace, the temperature in the upper part of the furnace is high, and the temperature in the lower part is low. , Combined with the analysis of the oxidation degree of the crack position, the roll has high temperature and long-term overburning during the heating and heating process. The high temperature will inevitably increase the upper and lower temperature gradient, which may cause internal stress in the roll structure, form micro-cracks, and gradually develop and form cracks.
The roll structure is long in the axial direction and large in cross-section. The thermal conductivity of the semi-steel roll is only half of that of 45 steel, and the thermal conductivity is poor; during casting, the temperature of the riser end is high, and the temperature of the lower end is low, and there is an objective upper and lower temperature gradient, and its axial tension The stress is far greater than the tangential and radial stress, which may cause transverse cracks in the roll body; when blowing and cooling, the air volume of the fan is small, and the adjustment of the blowing position is not appropriate so that the wind cover roll body is incomplete and the roll is partially expanded. Or inconsistent shrinkage, it may also cause transverse cracks in the roll body; small shoulder chamfers or radial expansion during processing. Hot unpacking was adopted, and the heating rate was strictly controlled, especially the heating rate after 680 °C was as slow as possible, the holding time at 680 °C was extended to 16 h, and the heating rate was modified to be less than or equal to 100 °C. In the heating and heat preservation stage, according to the temperature changes in the furnace, adjust the control temperature in time; the surface of the roller body is covered with aluminum silicate fiber felt for protection; adjust the height of the fan to align it with the center line of the roller body or transfer it to a spray mixer for rotary blowing .
4) Sticky sand
Roll sand sticking not only affects the appearance quality of castings but even causes scrap. The molten metal penetrates into the sand mold or the gap between the sand cores, sinters with the sand and adheres to the surface of the roll, or the metal oxide generated by the chemical reaction of the molten metal reacts with the molding material to form the iron silicate scum with strong adhesion to form sticky sand. Ensure that the dry strength of the sand mold is above 70 MPa, and the pouring superheat is 50 °C~60 °C; ensure that the purity of the SiO2 of the quartz sand is above 95%, and the sand particle size is 40 mesh ~ 100 mesh; use alcohol-based zircon refractory coating, coating thickness 2mm ~ 3 mm.
Usually, some smooth holes that are too small and different are often seen inside and on the surface of the roll, and the common ones are invasive pores, precipitation pores, and subcutaneous pores. There are many reasons for the occurrence of pores, such as the charge is not dry or contains many oxides and impurities; too much water in the molding sand or too much water when pulling the mold and repairing; insufficient drying of the core or blockage of the core ventilation holes; pouring The temperature is too low or the pouring speed is too fast, etc. For invasive pores, the content of gas-generating substances in molding sand or core sand should be controlled as soon as possible to reduce the amount of gas-generating, and the moisture content of wet molding sand should be reduced to ensure the dryness of the sand core. Preventive control of subcutaneous pores, including an appropriate increase in pouring temperature. Exhaust system, considering the optimization of the design of the riser system, the exhaust system layout is reasonable, so that the cavity and sand core can be exhausted smoothly, which is beneficial to prevent defects such as pores and sand sticking. Low air volume and good ventilation.
6) Sand holes and slag holes
Causes of sand holes and slag holes: the strength of the molding sand is too low, or the sand mold and the core are not strong enough, the sand mold is partially damaged when the box is closed, the pouring system is unreasonable, and the loose sand in the cavity or gate is not cleaned when the mold is closed. The shape is irregular and the defect is filled with inclusions. Try to improve the strength of the molding sand and the firmness and compactness of the sand mold, reduce the burr of the sand core, so as to prevent the phenomenon of sand washing; clean the floating sand on the surface of the cavity and the sand core before molding, and strictly avoid the molten steel Excessive scour force of the mold wall.
7) The hardness is not suitable
The flat roll is subjected to a rough opening and then quenched. The matrix structure of the adamite rolls determines the wear resistance and accident resistance of the roll. The microstructure of the roll can basically be regarded as a mixture of cementite and ferrite, especially for high-carbon materials. Although the amount of cementite is required to ensure its hardness and wear resistance, it must be controlled during the solidification process. The development of the carbon body network cuts and spheroidizes the as-cast carbide network to avoid being too brittle. By optimizing and improving the heat treatment process, the hardness uniformity of the roll is satisfied.