How to select and use rare earth magnesium nodulizer

In the field of industrial production, the main types of nodulizing agents are magnesium silicon alloy, rare earth magnesium silicon alloy, calcium alloy (mostly used in Japan), nickel magnesium alloy, pure magnesium alloy, rare earth alloy (mainly light rare earth with cerium and lanthanum and heavy rare earth with yttrium), etc. At present, rare earth magnesium ferrosilicon alloy is the most widely used alloy in the world. In addition, single light rare earth nodulizer (mainly cerium and lanthanum), single heavy rare earth nodulizer (mainly yttrium), barium containing nodulizer, antimony containing nodulizer, bismuth containing nodulizer, copper containing nodulizer, etc. are derived.

Nodulizing agent is an essential and important material for the production of nodular cast iron, and its quality directly affects the quality of the produced nodular cast iron, such as the nodulizing rate of castings, the size of graphite balls, and the tendency of castings to produce shrinkage porosity, porosity, slag inclusion, etc. In the production of nodular cast iron, only by fully understanding the role of various elements in the nodulizing agent, and then according to the actual production conditions and technical requirements of castings, reasonable selection and correct use of the nodulizing agent, can we ensure the production of high-quality nodular cast iron.

1、 Elements and characteristics of nodulizer

At present, the commonly used rare earth magnesium ferrosilicon nodulizer contains elements magnesium, rare earth, calcium, some iron, silicon, and a small amount of manganese, aluminum, titanium, etc. The composition of nodulizing agent is related to nodulizing treatment method and molten iron conditions. In this paper, the most commonly used rare earth ferrosilicon magnesium nodulizer is taken as an example for analysis.

1. Content of spheroidizing elements

Spheroidizing element is an element that can convert flake graphite in molten iron into spherical graphite. The transformation ability of almost all elements in the periodic table has been studied. It is concluded that magnesium is the best and most important spheroidizing element. In addition, cerium, lanthanum, calcium and yttrium can also be used as auxiliary spheroidizing elements in some cases.

The residual amount of spheroidizing elements in nodular iron castings is related to the casting size, wall thickness and sulfur content of the primary iron melt. To ensure the stability of graphite spheroidizing and spheroidizing, the effective residual magnesium content should be ＞ 0.030%. In order to ensure the necessary recovery rate of magnesium, the magnesium content of magnesium alloy nodulizer is usually less than 10%. When selecting the magnesium content of nodulizer, the nodulizer with slightly lower Mg (5% ～ 6% Mg) is generally selected when nodulizing molten iron at high temperature (1500 ～ 1550 ℃), and the nodulizer with slightly higher Mg (6% ～ 8% Mg) is selected when nodulizing molten iron at low temperature (1400 ～ 1450 ℃), so as to control the stability of nodulizing reaction and obtain appropriate residual magnesium content. In actual production, when the temperature difference is not very large, the workshop usually does not distinguish so strictly for the convenience of operation. Generally, the method of reasonable covering the nodulizing agent and controlling the amount of addition is used to control the stability of nodulizing explosion, so as to obtain the appropriate amount of residual magnesium.

The spheroidizing ability of rare earth elements is inferior to that of magnesium. The content of rare earth elements in domestic spheroidizing agents is generally divided into three grades: high (7% ～ 9%), medium (4% ～ 6%) and low (1% ～ 3%). In contrast, a large number of foreign nodular cast iron are smelted in electric furnace and adopt desulfurization process, so the nodulizer with low Mg (2.5% ～ 6.0%) and low RE (<2.0%) is mainly used. Therefore, the spheroidizing reaction is stable and the spheroidizing ink has a high roundness.

The rare earth can be divided into light rare earth and heavy rare earth. The domestic production of ordinary nodular cast iron mainly uses light rare earth nodulizer mainly composed of cerium (Ce) and lanthanum (La.

Calcium is generally the limited element in nodulizer, and proper amount (2% ～ 3% Ca is generally used for electric furnace) can control the absorption and reaction rate of nodulizer in molten iron. However, attention should be paid to excessive calcium, which will not only slow the melting of the nodulizer, but also lead to the development of graphite to worm shape, especially for large section ductile iron. Therefore, in the production of large section ductile iron, attention must be paid to the control of calcium in the nodulizer. A direct reflection of the low calcium in the nodulizer is that there is less slag in the ladle after nodulizing.

The purpose of barium in the nodulizer is to play the coordinating role of rare earth, magnesium and calcium, reduce the content of rare earth and magnesium, and make the nodulizing effect better. Barium, as a graphitized element, together with magnesium, can reduce the vapor pressure of magnesium at high temperatures, improve the absorption rate of magnesium, increase the number of graphite spheres per unit volume of ductile iron, strengthen the inoculation effect, and inhibit the formation of carbides.

2. Iron and silicon content

Silicon and iron are the basic components of the nodulizer, which are added when smelting alloys. Changing their content can adjust the density and melting point of the nodulizer. The silicon in the rare earth magnesium ferrosilicon nodulizer is generally 40%~50%, and the melting point is 1220 ℃. The melting point and density increase when Si is low and Fe is high. Si is too low (Fe must be high), so the spheroidizer is difficult to melt. In addition, when this spheroidizer is smelted, the terminal temperature is high, the ignition loss of Mg is large, and the MgO content may be high. When it is necessary to use more returning materials for nodular cast iron, the pressed low Si or “Si free” nodulizer should be used.

2、 Quality requirements of spheroidizing agent

The quality of spheroidizing agent is an important factor to determine the stable production of ductile iron. High quality spheroidizing agent must have: stable chemical composition and small fluctuation; The alloy has high purity and low MgO content; The alloy structure is dense; The particle size distribution is uniform and reasonable.

1. Chemical composition

The chemical composition of the nodulizer should be stable and have little fluctuation, otherwise it will seriously affect the quality stability of nodular cast iron production. Pay special attention to the fluctuation value of Mg and RE in the composition. The national standard stipulates that the allowable deviation of Mg and RE content in each brand is ± 1%. The primary content for evaluating the nodulizer is the deviation of the actual content of Mg and RE from the nominal content in the nodulizer. For generally good spheroidizing agent, the deviation shall be controlled within ± 0.2% ～ ± 0.3%. What needs to be said here is whether the RE type and proportion in the nodulizer supplied by the nodulizer manufacturers are stable and reliable, which is very important for nodular iron manufacturers. For example, a factory once made a comparative test, and the nodulizing agent is only RE, which is single Ce and Ce+La, respectively. As a result, the number of graphite balls containing Ce+La increased significantly, and the tendency of shrinkage porosity decreased.

2. MgO content

The spheroidizing elements in the spheroidizing agent must be active. If they are oxidized or vulcanized, they will lose their spheroidizing ability. Therefore, the spheroidizing elements shall not be oxidized or vulcanized during the production of spheroidizing agent. The stronger the degree of oxidation and vulcanization, the worse the spheroidizing effect of the spheroidizing agent. Therefore, special attention should be paid to the content of MgO in spheroidizing agent when producing spheroidizing agent.

The new national standard in 1993 stipulates that MgO is less than 1%, but according to the inspection results of nodulizing agent MgO produced by many high-quality nodulizing agent manufacturers and the actual use of ductile iron manufacturers, MgO control less than 0.50% can obtain the required effective magnesium, which is conducive to the stability of nodulizing quality. Sheng Da of Tsinghua University believes that if the magnesium oxide content in the nodulizer changes with the change of magnesium, it is inappropriate to control all nodulizers with a single 1.0% magnesium oxide content, and the control index should be changed to MgO%/Mg% ≤ 0.1+0.02.

3. Fracture density

By observing the fracture surface of nodulizer, the advantages and disadvantages of nodulizer can be directly distinguished. The high-quality spheroidizing agent is slightly yellow in grey blue, the fracture structure is dense, and the fracture surface is metallic luster (see Figure 1a). The section of low-quality spheroidizing agent is poor in density, with foreign matters such as gas shrinkage cavity and slag inclusion, and the section is gray (see Figure 1b).

The fracture surface of alloy ingot shall be dense, and there shall be no large scale shrinkage cavity, air hole and slag inclusion, otherwise the density of alloy will be reduced. The density of the alloy decreases, and the alloy has floated to the liquid surface before melting. The explosion is fierce, which not only affects the spheroidizing quality, but also affects the safety.

4. Particle size and density

The particle size of spheroidizing agent is an important factor affecting the reaction speed, which needs to be approved by the user. The particle size of nodulizer is mainly related to the amount of molten iron treated at one time (see the attached table). In addition, it is also related to many factors, such as the shape of molten iron ladle, the coverage of nodulizer, and the temperature of molten iron. Generally speaking, too large particles are easy to cause early explosion and floating; If the particles are too small, it is easy to form a dead bottom when the temperature of molten iron is insufficient. The above two conditions affect the yield of nodulizer and the stability of nodulation. Note that the size is too small, especially the powder spheroidizing agent cannot be used in production. In general, the nodulizing dose exceeding the standard shall be less than 10%.

The nodulizer density directly affects the yield of nodulizing elements. If the nodulizer density is small, it will float faster in molten iron. As for the nodulizer floating on the surface of molten iron, the gasification and oxidation burning loss of magnesium increases, especially when the temperature of molten iron is high and the nodulizer density is small, it is very easy to produce nodularization recession.

In addition, it has been proposed that the phase of nodulizer can be used as one of the criteria for quality inspection of nodulizer. As a kind of intermediate alloy, the chemical composition, smelting process, cooling rate of alloy ingot of nodulizer have corresponding relations with the metallography of nodulizer. Different metallographic structures will lead to different performance (mainly refers to the intensity of explosive reaction). The Si Mg phase in the 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 the actual use process, we found that the spheroidizing agent of the same composition is different in use and metallography. Figure 3 is the metallographic comparison of the spheroidizing agent. However, the main problem encountered at present is that there are no relevant provisions for sample preparation and metallographic evaluation.

3、 Use experience of spheroidizing agent

In the production of nodular cast iron, only by fully understanding the role of various elements in the nodulizer, and combining the actual production conditions and the technical requirements of castings, can the nodulizer be reasonably selected. Take the molten iron smelted by intermediate frequency furnace of our company as an example.

The molten iron smelted in the intermediate frequency furnace is generally low in sulfur (<0.03%). Our company uses high-quality pig iron and a large number of scrap steel, so the sulfur content of molten iron is lower (0.01%~0.02%) and the impurities in molten iron are less interfering elements. When producing thin castings (<50mm), spheroidizing agents containing wMg ＝ 5% ～ 5.5% and wRE ＝ 1.5% ～ 2.0% are selected; when producing medium thickness castings (50 ～ 100mm), spheroidizing agents containing wMg ＝ 5.5% ～ 6.0% and wRE ＝ 1.0% ～ 1.5% are selected; when producing thick and large castings (100 ～ 150mm), spheroidizing agents containing wMg ＝ 6.0% ～ 6.5% and wRE ＝ 0.5% ～ 1.0% are selected (some heavy rare earth spheroidizing agents can be added according to the situation).

In actual production, we found that in the production of medium thickness castings (50~100mm) and thick large section ductile iron (100~150mm), light rare earth wCe sometimes exceeds 0.006%, and broken graphite will appear at the center of the thickest hot joint, and the thick large section is more serious. In this case, we can avoid the occurrence of broken graphite by adding a small amount of antimony or bismuth to the molten iron, and the graphite ball will be refined, and the nodularity will be increased accordingly.

Although it is very important to select a suitable nodulizing agent, it is definitely not the only factor to stably produce ductile iron castings that meet the quality requirements, but a systematic project. The following matters should also be paid attention to:

(1) The stable and high-quality molten iron with low S and O content was obtained.

(2) Determine the appropriate nodulizing agent dosage and nodulizing element residue.

(3) Select reasonable chemical composition of molten iron.

(4) Correct spheroidizing inoculation process.

Taking the spheroidizing treatment of the commonly used plunge method as an example, the molten iron should be fast and cannot directly rush to the alloy when tapping. The height of the treatment package is generally 1.5~2 times of its diameter, and the short and thick package cannot be used. The bottom of the package should be equipped with a dam or a pit, and the space inside should be able to place all compacted spheroidizing agents and covering materials. The discharging temperature shall be reasonable. On the premise of ensuring the pouring temperature, the spheroidizing treatment temperature shall be reduced as much as possible. The time between the loading of spheroidizing agent and spheroidizing treatment as well as the transfer pouring time shall be shortened as far as possible, so as to ensure that the required residual spheroidizing elements are stably obtained during the spheroidizing process, and to prevent the occurrence of poor spheroidizing and spheroidizing recession. The components of inoculants need to be strictly screened, reasonably matched, and have high efficiency and long-term effect to ensure good inoculation effect and anti fading performance. Inoculant shall be added evenly and slowly along with the tapping iron flow, and the adding time shall account for more than 70% of the total tapping time.

4、 Conclusion

In addition to selecting appropriate nodulizing agent and ensuring the quality of nodulizing agent, the stable production of ductile iron requires correct use of nodulizing agent to ensure the production of stable ductile iron castings. This is a systematic project. The following points are summarized in detail.

(1) High quality liquid iron is the basis for stable production of high quality ductile iron.

(2) High quality nodulizer with proper composition is the key to produce ductile iron.

(3) The correct spheroidizing inoculation process is the guarantee.