The commonly used rare earth magnesium silicon iron spheroidizing agent currently contains elements such as magnesium, rare earth, calcium, as well as certain amounts of iron, silicon, and small amounts of manganese, aluminum, titanium, etc. The composition of the spheroidizing agent is related to the spheroidizing treatment method, molten iron conditions, etc. This article takes the most commonly used rare earth ferrosilicon magnesium spheroidizing agent as an example for analysis.
1. Content of spheroidizing elements
Spheroidizing elements are elements that can convert flake graphite in molten iron into spherical graphite. The conversion ability of almost all elements in the periodic table has been studied. Ultimately, it is believed that magnesium is the best and most important spheroidizing element, and in some cases, cerium, lanthanum, calcium, and yttrium can also be used as auxiliary spheroidizing elements.
The residual amount of spheroidizing elements in ductile iron castings is related to the size of the casting, the thickness of the wall, and the sulfur content of the original molten iron. To ensure the stability of graphite spheroidization and spheroidization, the effective residual magnesium content should be greater than 0.030%. In order to ensure the necessary recovery rate of magnesium, the magnesium content of magnesium alloy spheroidizing agents is often less than 10%. When selecting the magnesium content of the spheroidizing agent, a spheroidizing agent with slightly lower Mg content (5%~6% magnesium) is generally used for high-temperature spheroidizing iron liquid (1500-1550 ℃), and a spheroidizing agent with slightly higher Mg content (6%~8% magnesium) is used for low-temperature spheroidizing iron liquid (1400-1450 ℃). This can control the smoothness of the spheroidizing reaction and obtain an appropriate residual magnesium content. In actual production, when the temperature difference is not very large, the workshop often does not strictly distinguish between them for the convenience of operation. Generally, a reasonable coverage of spheroidizing agent and control of the amount added are used to control the smoothness of spheroidization explosion and obtain the appropriate residual magnesium content.
The spheroidization ability of rare earth elements is second to that of magnesium. The rare earth content in domestic spheroidizing agents is generally divided into three levels: high (7%~9%), medium (4%~6%), and low (1%~3%). In contrast, foreign ductile iron is extensively melted using electric furnaces and desulfurization processes, so low Mg (2.5% to 6.0%) and low RE (<2.0%) spheroidizing agents are mainly used. Therefore, the spheroidization reaction is stable and the roundness of the ink is relatively high.
Rare earths can be divided into light rare earths and heavy rare earths. In China, the production of ordinary ductile iron mainly uses cerium (Ce) and lanthanum (La) as light rare earth spheroidizing agents. However, with the continuous research on heavy rare earths, they have begun to be used in thick and large section ductile iron, and the use effect is good, which can effectively overcome the problems of spheroidization decline, strong section sensitivity, and low mechanical properties in the center of the section of thick section large ductile iron parts.
Calcium is generally a limiting element in spheroidizing agents, and an appropriate amount (usually 2% to 3% Ca is used in electric furnaces) can control the absorption and reaction rate of spheroidizing agents in molten iron. However, excessive calcium should be noted as not only does the spheroidizing agent melt slowly, but it can also cause graphite to develop into worm like structures, especially in large section ductile iron. Therefore, in the production of large section ductile iron, it is necessary to pay attention to the control of calcium in the spheroidizing agent. Another intuitive reflection of low calcium in the spheroidizing agent is the lack of slag in the ladle after spheroidization.
Barium is used in spheroidizing agents to enhance the coordination of rare earth, magnesium, and calcium elements, reduce the content of rare earth and magnesium, and improve the spheroidizing effect. Barium, as a graphitizing element, together with magnesium, can reduce the vapor pressure of magnesium at high temperatures, increase the absorption rate of magnesium, increase the number of graphite balls per unit volume of ductile iron, strengthen the inoculation effect, and suppress the formation of carbides.
2. Content of iron and silicon
Silicon and iron are the basic components of spheroidizing agents, which are added during alloy melting. Changing their content can adjust the density and melting point of the spheroidizing agent. The silicon in rare earth magnesium ferrosilicon spheroidizing agent is generally between 40% and 50%, with a melting point of 1220 ℃. If the Si is low and the Fe is high, the melting point increases and the density increases. Si is too low (Fe must be high), making it difficult for the spheroidizing agent to melt. Moreover, this spheroidizing agent has a high endpoint temperature during melting, resulting in significant loss of Mg and possibly higher MgO content. When it is necessary to use more ductile iron recycled materials, it is advisable to use pressed low Si or “Si free” spheroidizing agents.
2、 Quality requirements for spheroidizing agents
The quality of spheroidizing agent is an important factor determining whether ductile iron can be produced stably. High quality spheroidizing agents must have: stable chemical composition and minimal fluctuations; High alloy purity and low MgO content; The alloy structure is dense; Uniform and reasonable particle size distribution, etc.
1. Chemical composition
The chemical composition of the spheroidizing agent should be stable with minimal fluctuations, otherwise it will seriously affect the quality stability of ductile iron production. Pay special attention to the fluctuation values of Mg and RE in the composition. The national standard stipulates that the allowable deviation of Mg and RE content in various grades is ± 1%. The primary content for evaluating spheroidizing agents is the magnitude of the deviation between the actual content of Mg and RE in the spheroidizing agent and the nominal content. A generally good spheroidizing agent should have a deviation controlled within ± 0.2% to ± 0.3%. It should be noted that the stability and reliability of the RE type and proportion in the spheroidizing agent supplied by the spheroidizing agent manufacturer are crucial for ductile iron manufacturers. For example, a certain factory once conducted comparative experiments, and the spheroidizing agents were only different in RE, with single Ce and Ce+La, respectively. The results showed that the number of graphite spheres containing Ce+La increased significantly and the tendency to shrink decreased.
2. MgO content
The spheroidizing elements in the spheroidizing agent must be active. If they are oxidized or sulfided, they will lose their spheroidization ability. Therefore, when producing spheroidizing agents, it is necessary to avoid oxidation or sulfurization of spheroidizing elements. The stronger the degree of oxidation and vulcanization, the worse the spheroidization effect of the spheroidizing agent. Therefore, special attention should be paid to the content of MgO in the production of spheroidizing agents.
In 1993, the new national standard stipulated that MgO should be less than 1%. However, based on the inspection results of many high-quality spheroidizing agent manufacturers and the actual use of ductile iron manufacturers, controlling MgO to be less than 0.50% can obtain the required effective magnesium content, which is beneficial for the stability of spheroidizing quality. Sheng Da from Tsinghua University believes that the magnesium oxide content in spheroidizing agents varies with the change of magnesium, and it is inappropriate to control all spheroidizing agents with a single 1.0% magnesium oxide content. The control index should be changed to MgO%/Mg% ≤ 0.1+0.02.
3. Compaction degree of fracture surface
Observing the fracture surface of the spheroidizing agent can intuitively distinguish the quality of the spheroidizing agent. High quality spheroidizing agents have a slightly yellowish gray blue color, with a dense fracture structure and a metallic luster (see Figure 1a). Poor quality spheroidizing agents have poor cross-sectional density, with foreign objects such as gas shrinkage pores and slag inclusions, and a dull cross-section (see Figure 1b).
The fracture surface of the alloy ingot should be dense, without extensive shrinkage, porosity, and slag inclusion, otherwise the density of the alloy will decrease. The density of the alloy decreases, and the alloy floats to the liquid surface before melting, causing intense explosions that not only affect the quality of spheroidization but also affect safety.
4. Particle size and density
The particle size of the spheroidizing agent is an important factor affecting the reaction rate and requires user approval. The particle size of the spheroidizing agent is mainly related to the amount of molten iron processed in one step (see attached table). In addition, it is also related to many factors such as the shape of the molten iron package, the degree of coverage of the spheroidizing agent, and the temperature of the molten iron. Generally speaking, particles that are too large can cause premature explosions and floating; When the particle size is too small, it is highly likely to form a dead bottom when the temperature of the molten iron is insufficient. Both of the above situations affect the yield of spheroidizing agents and the stability of spheroidization. Note that the size is too small, especially for powdered spheroidizing agents, which cannot be used in production. In general, the excessive spheroidization dose should be less than 10%.
The density of spheroidizing agents directly affects the yield of spheroidizing elements. If the density of the spheroidizing agent is low, it will float faster in the molten iron. The spheroidizing agent floating on the surface of the molten iron increases the gasification and oxidation burn loss of magnesium, especially when the temperature of the molten iron is high and the density of the spheroidizing agent is low, it is extremely prone to spheroidization decay.
In addition, it has been suggested that the phase of the spheroidizing agent can be used as one of the quality inspection criteria for spheroidizing agents. As an intermediate alloy, there is a corresponding relationship between the chemical composition, melting process, cooling rate of the alloy ingot, and the metallographic structure of the spheroidizing agent. The performance of the spheroidizing agent will vary depending on the metallographic structure (mainly referring to the intensity of the explosive reaction). Si Mg phase in spheroidizing agent has many shapes, which can be generally divided into three categories: globular, globular and lath, as shown in Figure 2. Its shape, size, quantity, and distribution directly affect the release behavior of Mg in molten iron. In practical use, we found that spheroidizing agents with the same composition have differences in usage and metallography. Figure 3 shows the metallographic comparison of spheroidizing agents. But the main problem currently encountered is the lack of relevant regulations for sample preparation and metallographic evaluation