The Relationship Between Steel Casting Cracks And Inclusions In Steel

The inclusions generated during the smelting process of molten steel are one of the important causes of cracks in steel castings. In order to reduce the inclusions in the molten steel, during the smelting process, it is necessary to strengthen the smelting operations such as deoxidation, desulfurization, impurity removal, and degassing, and take necessary measures in the ladle behind the furnace, such as adding rare earths, etc. The shape of the inclusions can reduce the existence of inclusions and better eliminate the cracks of the steel castings.

1 Types and causes of inclusions in steel

Inclusions in steel mainly refer to non-metallic inclusions in steel. It is generally believed that non-metallic inclusions in steel often exist in the following forms: oxides: FeO, Fe2 O 3, M nO, Al2 O 3, SiO 2, M gO, etc. ; Sulfides: M nS, FeS, etc.; Silicates: FeSiO 4, M nSiO 4, FeO ·Al2 O3 ·SiO2, etc.; Nitrides: AlN, Si3 N 4 and so on.

Non-metallic inclusions in steel come from two aspects: First, they are generated during the smelting process, that is, the deoxidation products of ferroalloys are added during tapping and the secondary oxidation products of molten steel and air during the casting process, which are called endogenous inclusions. Such inclusions are generally fine particles and uniformly distributed in the steel; second, they are brought in from the outside due to various reasons, called foreign inclusions. Such inclusions are often irregular in shape, large in size and unevenly distributed, which is the cause of cracks. The main reason is that it is more harmful to steel.

Endogenous inclusions are mainly produced in the following situations:

• ①During the smelting process, the deoxidation products were not completely eliminated, or the temperature dropped during the pouring process, and the deoxidation products generated by the continued reaction did not have time to float and remain in the molten steel. Some of them exist in the matrix structure of the steel as small particles, and some Then it aggregates into large particles of Al2O3), and some exist in the steel in a solid solution state (such as M nO, F eO);
• ②In the process of tapping and pouring, the molten steel is oxidized in contact with air, and oxygen and steel The middle elements combine to form secondary oxides and stay in the molten steel; during the solidification of the molten steel, low melting point FeS, FeO, etc. are finally precipitated in the grain boundaries and between the dendrites due to the “selective crystallization” of the molten steel.

Foreign inclusions: This kind of inclusions are mainly involved in the sand, slag and mold slag brought by the raw materials. The refractory materials of the pouring system are scoured and eroded by the molten steel. They are retained in the molten steel and are mostly large particle inclusions. Things.

Non-metallic inclusions are dissolved in molten steel at high temperatures, or exist alone in molten steel, but as the temperature drops and the composition, gas pressure and other conditions change, the original inclusions dissolved in molten steel will be The independent phases are separated and gathered on the grain boundaries during the crystallization process, and become the tiny units that cut the connection between the cast steel matrix and form the initial source of cracks, thus forming potential cracks.

2 The relationship between main inclusions and cast steel cracks and measures to reduce

Among non-metallic inclusions, the main cause of cracks in steel castings is sulfide inclusions, and it often interacts with other factors to increase the tendency of steel castings to crack. In cast steel, sulfide inclusions are divided into three categories:

• Type Ⅰ-spherical;
• Type Ⅱ-point chain intercrystalline film;
• Type Ⅲ-randomly distributed sharp angle.
• Among them, type II inclusions are the most harmful to steel, followed by type III, and type I is the least.

The sulfide inclusions are related to the degree of deoxidation of the steel and the amount of residual aluminum in the steel. When the amount of aluminum solid solution is low and the oxygen residue is small, Type I inclusions can be obtained.

The deoxidizer has a great influence on the formation of inclusions and the performance of steel. The deoxidizing effect of the composite deoxidizer is better than that of a single deoxidizer. This is because the inclusions formed by the composite deoxidizer are larger, which are easy to float and remove. If the molten steel is insufficiently deoxidized, pore defects are likely to occur and cracks are likely to occur in the steel castings. However, the amount of aluminum used for final deoxidation is just enough for deoxidation and there is no residue. The solid solution content of aluminum is low, and the residual oxygen is small, which will produce type II inclusions. Generally, if excessive aluminum is used for deoxidation, type III inclusions will be obtained. It is worth noting that if excessive aluminum is used, more aluminum nitride inclusions will be formed and precipitate along the grain boundaries, leading to "rock-like" brittle fractures and deteriorating the performance of the steel. Therefore, it is unreasonable to use excessive aluminum for deoxidation. The amount of aluminum added during steelmaking and the amount of residual aluminum in the steel can neither be too low nor too high.

Deoxidation with aluminum is a widely used method for steel deoxidation. Two kinds of deoxidation processes are usually used in industrial production, one is aluminum deoxidation process, and the other is controlled aluminum deoxidation process. The former is to use aluminum to completely remove the dissolved oxygen in the steel, and then to remove as many Al2O3 inclusions as possible through various methods of stirring; the latter is to use only silicomanganese for coarse deoxidation, and strictly control the aluminum and aluminum in the steel. Calcium content in order to control the composition, nature and morphology of the oxide inclusions precipitated in the steel. The primary deoxidation rate of the former is greater than 90, and the deoxidation product is mainly Al2O3; the amount of deoxidation products precipitated by the latter deoxidation is greatly reduced, and the primary deoxidation product is mainly SiO2.

The foreign inclusions can be removed according to their source by taking corresponding measures, while the endogenous inclusions need to be controlled by the new technology of deoxidation process and calcium treatment process.

During ladle refining, blowing more and smaller argon bubbles into the molten steel is beneficial to the removal of the first deoxidation products.

In order to better remove the inclusions in the steel and reduce cracks, the smelting operation takes the following measures.

• (1) Prepare raw materials well to prevent foreign inclusions.
• (2) Adopt reasonable steelmaking processes: such as adopting reasonable oxygen blowing and power distribution processes, ensuring a certain decarburization speed to make inclusions float up, and maintaining good furnace conditions.
• (3) Use composite deoxidizer instead of single deoxidizer.
• (4) Rare earths are added to the ladle behind the furnace to change the shape of inclusions and reduce inclusions to reduce the tendency of steel castings to crack and increase the fluidity of molten steel.
• (5) In order to facilitate the removal of inclusions, in addition to ensuring sufficient molten steel temperature, the molten steel should be properly placed in the ladle after tapping.

In addition, the use of high-quality refractory materials to ensure that the pouring system is clean or the use of filters are also important measures to reduce inclusions.

3 Rare earth composite modification treatment to remove inclusions in steel and adding methods

Adding rare earth has a good effect on removing inclusions in steel. Rare earth is mainly used to control sulfide in steel, and it can deoxidize and desulfurize.

In the as-cast condition, the M nS inclusions in the steel are elliptical or approximately circular. The larger elliptical inclusions are composite inclusions formed by M nO as the core and the outer layer surrounded by M nS or M n-S-O. After adding the rare earth, the distribution and composition of the as-cast inclusions have changed, and the MnS inclusions are replaced by approximately spherical fine and dispersed rare earth inclusions. In order to reduce rare earths well, rare earth oxides should be mixed with strong reducing agents (Ca-Si, Ca-B) and then added to the molten steel, so as to play the role of modification treatment. With proper modification treatment, sulfide inclusions can form spherical inclusions with high melting point, low plasticity and stable thermodynamic properties. It can be said that rare earth and calcium are good desulfurizers and good modifiers for sulfide inclusions. The use of RE-Ca composite treatment can better deoxidize, desulfurize, purify, and deteriorate, control the shape and distribution of non-metallic inclusions, and improve the comprehensive mechanical properties of cast low-alloy steel.

In order to further explore the effect of rare earth on the properties of cast steel, the use of rare earth composite modification treatment in ZG 35CrM o steel was tested. The rare earth alloy grade used is YX 20, containing 20.53 RE, 40.95 Si; Si-Ca composition Contains 26.45 Ca and 57.07 Si.

The composite treatment adopts the method of adding in the ladle. When the rare earth is added, the molten steel must be fully deoxidized to prevent burning when the rare earth is added, and to prevent it from reacting with the ladle refractory. Rare earths should be baked well before being added. The specific method is to determine the amount of RE and Si-Ca according to the amount of molten steel in the ladle and how much sulfur it contains, break RE and Si-Ca into small pieces and mix them evenly, cover them with iron sheets, and quickly insert aluminum for final deoxidation. The molten steel after the slag is constantly agitated, and the treatment temperature is maintained within the range of 1 600 ～ 1 650 ℃, then the slag is removed, and it is left standing for 1 to 2 minutes for pouring.

According to different recovery rates, the appropriate value of RE/S for the residual rare earth and sulfur content in molten steel is controlled to obtain a good deterioration effect. Studies have proved that when RE/S ≈3, M nS inclusions can be completely deteriorated; when RE/S <3, only partial deterioration can be achieved; when S ≈0.02 in steel, RE/S = 1.8 ～ Deterioration effect is best when 2.5 hours.

The role of rare earth composite modification treatment is mainly to purify molten steel and control the shape of inclusions, reduce inclusions, and at the same time eliminate Widmanstatten structure, refine grains and microalloy. That is, after adding the rare earth composite agent, it can not only deoxidize and desulfurize, but also when RE/S ≥3 ～ 6, MnS will become spherical inclusions, thereby reducing the harm of sulfur and preventing or reducing hot cracking. Rare earth is effective in changing the inclusions in molten steel. The shape of the object reduces cracks and the effect is obvious.

The gears produced with ZG 35C rM o steel have a high scrap rate before the compound modification treatment, but after the compound modification treatment, the scrap rate is reduced by more than 40%, which has achieved obvious results and obtained good economic benefits.

4 Conclusion

• (1) Inclusions in molten steel are one of the main causes of cracks in cast steel. Different inclusions have different ways of producing cracks.
• (2) In order to reduce non-metallic inclusions in molten steel and prevent cracks in steel castings, it is necessary to do a good job in the aspects of raw material preparation, smelting process, deoxidation operation, ladle standing, and modification treatment.
• (3) Using rare earth and calcium combined modifiers for composite modification of molten steel is an effective method to reduce non-metallic inclusions in molten steel, change their shape, and reduce cracks in steel castings.

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