Aluminum is a powerful deoxidizer. Adding an appropriate amount of aluminum during steelmaking is an important measure to improve the quality of molten steel. Aluminum exists in three states in steel, namely, solid solution aluminum. 2 aluminum nitride. 3 Alumina and aluminate inclusions. The sum of these three substances is the total residual aluminum content of the steel. The solid solution aluminum and aluminum nitride are acid-soluble aluminum in chemical analysis, and the aluminum oxide compound is insoluble in acid, and obvious inclusions are present in the steel. When the amount of aluminum is more than 0. 02% ～0, the amount of aluminum is generally controlled. . 04 %. The quality of steel is better, the pores and inclusions are less, and cracks are less; on the contrary, the crack resistance is poor and the crack is much. Oxygen exists in the form of FeO in the molten steel. The more oxygen content in the molten steel, the more the FeO precipitates in the grain boundary of the steel due to the decrease in solubility during the solidification of the molten steel, reducing the crack resistance of the steel. Therefore, the oxygen content of the molten steel is strictly controlled to be less than 0.006 % (volume fraction).
2. Gating system
The bolster pouring system is arranged as shown in the drawing, the inner runner is arranged at one end of the bolster, and the solidification sequence after the molten steel is poured is first solidified away from the other end of the inner runner, and then gradually solidified toward the inner runner. The solidification sequence tends to weakly sequential solidification, which is characterized by large temperature difference during solidification of various parts of the casting, and the generated stress is large, and the cavity near the position of the inner runner I is baked for a long time in high temperature molten steel. Severe chemical sticking, solidification hinders the shrinkage of the casting, causing thermal cracking at this location.
3. Casting structure
Since the bolster belongs to the thin-walled box-like structure, the casting shape and the core are made of water-glass silica sand. The inner cavity is mainly composed of 1 # and 2 # a total of 3 large sand cores. The water glass sand type and the sand core have poor retractability. Therefore, when the molten steel solidifies, it generates a large resistance and has a direct influence on the generation of the broach crack.
4. Pouring temperature
For thin-walled castings, increasing the pouring temperature can improve the filling ability, but as the casting temperature increases, the primary crystal structure formed during the solidification of the casting is coarse, and when the coarse equiaxed crystals are connected to each other, hot cracking occurs in the casting. The tendency is great because the coarser the equiaxed crystal, the lower the high temperature strength; in addition, when cracks occur between the crystals, the filling of the liquid metal is not obtained to heal. Therefore, the casting temperature of the casting is strictly controlled in operation.
Third, preventive measures
Smelting
(1) Melting is performed by an alkaline arc furnace oxidation process. 01 % 至0. 02 %。 The oxidation method is carried out with ore-oxygen combination to ensure a smooth dephosphorization, degassing, de-intercalation, the oxidation should be carried out under good boiling conditions, the decarburization rate is 0. 01% ~ 0. 02% per minute. At the end of the oxidation period, when the carbon content is less than the lower limit of the component of 0.08 % (mass fraction), C, Mn, and P are analyzed by thorough stirring. Enter the net boiling, the time should be ≥ 15min. After the net boiling for 5 minutes, when the temperature of the molten pool is ≥1620 °C, ferromanganese is added for pre-deoxidation, so that the molten pool w Mn is >0.15%.
(2) In the initial stage of the reduction period, the aluminum is pre-deoxidized by the addition of pure aluminum. When the aluminum is finally deoxidized, the amount of the steel solution wAl is 0.22% to 0.04%.
(3) Pass argon gas into the ladle for in-pack refining. The argon pressure is generally 0. 4 ~ 0. 6MPa, the time is 5 ~ 8min, the molten steel should not be exposed.
(4) Control the pouring temperature, measure the temperature before pouring, calm the molten steel, and control the pouring temperature between 1540 and 1580 °C.
(5) Loosen the box in time after pouring the casting to reduce the shrinkage resistance.
2. Gating system
The position of the gate is changed, and the molten steel is introduced into the cavity from the middle position of the bolster during pouring. The solidification sequence of the casting tends to solidify at the same time, which can reduce the temperature difference between the various parts of the casting, thereby reducing the stress and preventing the occurrence of sand at the I position.
3. Set cold iron and anti-cracking
The arc outer cold iron is arranged at the position of the arc II, so that the solidification speed of the place is equalized with the solidification speed of the adjacent section, the thickness of the cold iron is equal to the wall thickness of the casting, and three anti-cracking ribs are arranged at the position. The thickness is equal to 1/3 of the wall thickness of the casting. After the anti-cracking rib is set, since the solidification is fast and the strength is established earlier, it can withstand a large tensile stress, which is favorable for preventing cracks at the position.
4. Using chromite ore
When modeling, place a layer of chromite ore with a thickness of 30mm on the surface of the arc III. Since the thermal conductivity of the chromite ore is several times larger than that of the silica sand, the solidification speed at the place can be balanced with the solidification speed of the adjacent section, which is advantageous for preventing cracks at the position. Technical conditions of chromite ore: Cr2O3 ≥ 45 %, SiO2 ≤ 5 %, CaO ≤ 1%, ignition loss ≤ 1%, refractoriness 1600 ~ 1800 °C.
Fourth, the effect
The improved procedural measures were used to produce 100 pieces of bolsters. The original production records were compared, and the bolster technical quality indexes produced before and after the improvement of the process measures were compared. The results are shown in the attached table.
It can be seen from the table that after the implementation of the improved process measures, the technical indexes of the bolster and the product yield have been significantly improved.
V. Conclusion
In the case of a certain level of process equipment, the key to eliminating the thermal crack of the bolster to improve its quality is to improve the technical conditions. By continuously summing up the experience and perfecting the technical conditions, we have basically eliminated the hot crack defects generated in the KF60 self-turning bolster casting production process, and the product yield has been greatly improved, and significant economic and social benefits have been achieved.
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