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Cracking of cast steel parts and the prevention and control measures of inclusions in steel:
Steel in the smelting process of inclusions, is one of the important reasons for the cracking of cast steel parts. In order to reduce the inclusions in the steel, in the smelting process, to strengthen the deoxidation, desulfurization, debris removal, de-gassing and other smelting operations, and in the ladle after the furnace to take the necessary measures, such as adding rare earths and other densification process to change the shape of inclusions, in order to reduce the presence of inclusions, better eliminate the cracking of cast steel, xinyuanzhu group combined with the production process of cast steel, to make The following summary hopes to help the casting enterprises
1、 the types of inclusions in steel and the causes
Inclusions in steel mainly refers to non-metallic inclusions in steel, it is generally believed that non-metallic inclusions in steel often exist in the form of
Oxide: F eO, Fe2 O 3, M nO, Al2 O 3, SiO 2, M gO, etc.;
Sulfides: MnS, FeS, etc;
Silicates: FeSiO 4, MnSiO 4, FeO-Al2 O3 -SiO2, etc;
Nitrides: A lN, Si3 N 4, etc.
1.1 Sources of non-metallic inclusions in steel:
1.11 with the smelting process, that is, when the steel is added to the deoxidation products of ferroalloys and pouring process of steel and air in the secondary oxidation products, called endogenous inclusions, such inclusions are generally fine particles, uniform distribution in the steel
1.12 for various reasons brought in from outside, called foreign inclusions, such inclusions are more irregular in shape, size and uneven distribution, is the main cause of cracking, the greater the harm to steel.
1.2 endogenous inclusions are mainly generated in the following cases :
1.21 in the smelting process, deoxidation products are not all excluded, or pouring process temperature drop, continue to react to generate deoxidation products to float residual in the steel, some of them with small mass in the steel matrix organization, some are agglomerated into large particles Al2 O 3 ), some are in a solid state in the steel (such as M nO, F eO)
1.22 in the steel and pouring process, the steel and air contact oxidation, oxygen and steel elements combined to form secondary oxides in the steel; in the steel solidification process, the low melting point of FeS, FeO, etc. due to the steel "selective crystallization", and finally in the grain boundaries and dendrites between precipitation.
1.23 foreign inclusions: these inclusions are mainly brought in by the raw material sediment, molten slag and protection slag, etc. involved in the pouring system of refractory by the steel scouring, leaching, etc. into, retained in the steel, mostly large particles inclusions.
Non-metallic inclusions at high temperatures or dissolved in the molten steel, or alone in the molten steel, but as the temperature drops and composition, gas pressure and other conditions change, the original inclusions dissolved in the molten steel, will be separated from the independent phase, gathered in the crystallization process on the grain boundaries, become a tiny unit to cut the connection of the cast steel matrix, the initial source of the formation of cracks, thus forming a potential source of cracking.
2、the main inclusions and cast steel cracking relationship and measures to reduce
In the non-metallic inclusions, the main reason for cracking cast steel is sulfide inclusions, and it often works together with other factors, so that the cast steel cracking tendency increases. Cast steel, sulfide inclusions are divided into three categories:
2.11 type, spherical
2.12 class, point chain intergranular film-like
2.13 category, arbitrary distribution of sharp-angle shape.
Among them, 2.12 types of inclusions are the most harmful to steel, 2.13 types are the next, 2.11 types are smaller. and sulfide inclusions and the degree of deoxidation of steel and the amount of residual aluminum in steel. Aluminum solid solution is low, when the oxygen residue is small, can be 1 type of inclusions.
Deoxidizer on the formation of inclusions and steel properties have a greater impact. Compound deoxidizer deoxidation effect is better than a single deoxidizer, this is because the compound deoxidizer formation of inclusions larger, easy to remove the floating. If the steel deoxidation is not enough, easy to appear porosity class defects, easy to make the cast steel crack. But the final deoxidation with aluminum is just enough to deoxidation and no residual, aluminum solid solution is low, oxygen residual less, will produce 2 types of inclusions, generally with excess aluminum deoxidation, will get 3 types of inclusions. It is worth noting that, such as the use of excess aluminum will form more aluminum nitride inclusions along the grain boundary precipitation, resulting in "rock-like" brittle fracture, deteriorating the performance of steel. Therefore, the use of excessive aluminum deoxidation is not reasonable. The amount of aluminum added to steel and the amount of residual aluminum in the steel can neither be too low nor too high.
2.2 Deoxidation with aluminum is a widely used method of steel deoxidation. Two deoxidation processes are usually used in industrial production
2.21 Aluminized deoxidation process,
2.22 controlled aluminum deoxidation process.
The former is to use aluminum to completely remove the dissolved oxygen in the steel, and then through a variety of ways to remove Al2 O3 inclusions as much as possible; the latter is only with silicomanganese crude deoxidation, and strict control of aluminum and calcium content in steel, in order to control the composition of the precipitated oxide inclusions, nature and form. The former''''s primary deoxidation rate is greater than 90, deoxidation products are mainly Al2 O3; the latter is a deoxidation precipitation of deoxidation products is greatly reduced, the primary deoxidation products are mainly SiO2.
2.3 foreign inclusions can be removed according to their sources to take appropriate measures, but the endogenous inclusions need to be controlled through the deoxidation process and new technology of calcium treatment process.
The removal of primary deoxidation products is facilitated by blowing a larger number of smaller argon bubbles into the steel during ladle refining.
In order to better remove inclusions in steel, reduce cracking, smelting operations to take the following measures.
2.31 Prepare the raw material well to prevent foreign inclusions.
2.32 to take a reasonable steelmaking process: such as taking a reasonable oxygen blowing, distribution process, to ensure that a certain decarburization rate so that the inclusions float, maintain good furnace conditions.
2.33 to take a composite deoxidizer instead of a single deoxidizer.
2.34 after the furnace ladle with rare earths, change the shape of inclusions, reduce inclusions, in order to reduce the cracking tendency of cast steel, increase the liquidity of the steel.
2.35 In order to facilitate the exclusion of inclusions on the float, in addition to ensuring sufficient steel temperature, the steel ladle after the steel to be properly resting.
In addition, the use of high-quality refractory materials, to ensure that the pouring system clean or the use of filters, is also an important measure to reduce inclusions.
3、rare earth composite densification treatment to remove inclusions in steel and the addition of methods
The addition of rare earths to remove inclusions in steel has a good effect, rare earths in steel is mainly used to control sulfides, and can deoxidation, desulfurization .
In the as-cast condition, MnS inclusions in steel are elliptical or nearly circular in shape. The larger elliptical inclusions are composite inclusions with M nO as the core and M nS or M n-S-O as the outer layer. After the addition of rare earths, the distribution and composition of the as-cast inclusions changed, and the MnS inclusions were replaced by fine, diffuse inclusions with a nearly spherical shape. In order to reduce the rare earths well, the rare earth oxides should be mixed with strong reducing agents (Ca-Si, Ca-B) and added to the molten steel, so that the effect of the densification treatment can be achieved. With proper densification treatment, the sulfide inclusions can be formed into spherical inclusions with high melting point, low plasticity and more stable thermodynamic properties. It can be said that rare earths and calcium are good desulfurization agents, and good denaturing agents for sulfide inclusions. The RE-Ca composite treatment can better deoxidize, desulfurize, purify, densify, control the morphology and distribution of non-metallic inclusions, and improve the comprehensive mechanical properties of cast low-alloy steel.
In order to further investigate the effect of rare earths on the properties of cast steel, a test was conducted on ZG 35CrMo steel using rare earth composite densification treatment with rare earth alloy grade YX 20, containing 20.53 RE, 40.95 Si; Si-Ca composition containing 26.45 Ca, 57.07 Si.
When adding rare earths, the steel must be fully deoxidized to prevent burnout and reaction with the ladle refractory. Rare earths should be well baked before adding. Specific practice is to determine the amount of RE and Si-Ca according to the amount of steel and its sulfur content in the ladle, the RE and Si-Ca broken into small pieces mixed well, covered with iron, quickly inserted into the steel with aluminum for the final deoxidation slagging, and keep stirring, processing temperature is maintained in the range of 1 600 ~ 1 650 ℃, and then slagging, Then the slag is stripped and left to stand for 1 to 2 min before pouring.
According to different recovery rates, control the appropriate value of rare earth residue and sulfur content in the steel RE / S, can obtain a good densification effect. It is proved that when RE/S ≈ 3, the MnS inclusions can be completely densified; when RE/S < 3, only partially densified; when the steel S ≈ 0.02, RE/S = 1.8 ~ 2.5 is the best densification effect.
The chemical composition of the steel after densification, the sulfur content in the steel after RE and RE-Ca treatment decreased significantly, while the recovery of RE in the steel increased with the increase of RE addition. The mechanical properties of the steel molten by the densification treatment were significantly improved, with the strength increased by more than 11, and the overall performance of the RE-Ca composite treatment was better, with the impact toughness increased by about 26.
The role of rare-earth composite densification treatment is mainly in the purification of steel and control the inclusions form, reduce inclusions, but also to eliminate the Weiss organization, refining grain and micro-alloying effect. That is, the addition of rare earth compound not only deoxidation and sulfur, and in RE / S ≥ 3 ~ 6, MnS will become spherical inclusions, thereby reducing the harm of sulfur, prevent or reduce thermal cracking, rare earth for changing the morphology of inclusions in the steel, reduce cracking, the effect is very obvious.
The gears produced by ZG 35C rM o steel, the scrap rate is very high before the composite densification treatment, but after the composite densification treatment, the scrap rate is reduced by more than 40%, which has achieved obvious results and obtained good economic benefits.
4、Conclusion
4.1 inclusions in the steel is one of the main reasons for the cracking of cast steel, different inclusions, the way to produce cracks are different.
4.2 In order to reduce the non-metallic inclusions in the steel and prevent the cast steel cracking, from raw material preparation, smelting process, deoxidation operation, ladle resting, densification treatment and other aspects, do a good job.
4.3 the use of rare earth and calcium combined denaturing agent for steel composite densification treatment, is to reduce the non-metallic inclusions in the molten steel, change its shape, reduce the effective method of cast steel cracks.
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