High-Strength Gray Cast Iron Smelting Technology

Publish Time: 06/20 2021 Author: Site Editor Visit: 15010

This article introduces how to obtain high-strength gray cast iron smelting technology under the conditions of higher carbon equivalent and better machining performance requirements in the electric furnace smelting process, and how to control the trace elements of the material.

Key words: gray cast iron, carbon equivalent, mechanical properties, processing properties, trace elements

The traditional gray cast iron smelting control direction is low-carbon high-strength cast iron (C: 2.7~3.0, Si: 2.0~2.3, Mn: 0.9~1.3). Although such materials can meet the requirements of material mechanical properties, their casting performance and processing The performance is poor. With the company’s market development and expansion, more and more casting products with high difficulty and high technical quality requirements are included in the MINGHE production sequence, especially when MINGHE uses the power frequency electric furnace smelting process to replace the cupola smelting process.

Obtaining high-carbon equivalent high-strength cast iron under electric furnace smelting conditions to meet customer order requirements was a research topic at that time. This article describes the production technology of high-strength gray cast iron under electric furnace smelting conditions.

The Factors Of Affecting Material Performance

1.1 The effect of carbon equivalent on material properties

The main factors that determine the properties of gray cast iron are the graphite morphology and the properties of the metal matrix. When the carbon equivalent (CE=C+1/3Si) is high, the amount of graphite increases, and the shape of graphite deteriorates when the incubation conditions are not good or there are trace harmful elements. Such graphite reduces the effective area of the metal matrix that can bear the load, and causes stress concentration when bearing the load, so that the strength of the metal matrix can not be used normally, thereby reducing the strength of the cast iron. Among the materials, pearlite has good strength and hardness, while ferrite has a softer base and lower strength. As the amount of C and Si increases, the amount of pearlite will decrease and the amount of ferrite will increase. Therefore, the increase in carbon equivalent will affect the tensile strength of cast iron castings and the hardness of the casting entity in both the graphite shape and matrix structure. In the control of the smelting process, the control of the carbon equivalent is a very important factor to solve the material performance.

1.2 The influence of alloying elements on material properties

The alloying elements in gray cast iron mainly refer to Mn, Cr, Cu, Sn, Mo and other elements that promote the formation of pearlite. The content of these elements will directly affect the content of pearlite. At the same time, due to the addition of alloying elements, it is refined to a certain extent. The addition of graphite reduces or even disappears the amount of ferrite in the matrix, while pearlite is refined to a certain extent, and the ferrite in it is solid solution strengthened due to a certain amount of alloying elements, so that the cast iron always has a higher The strength performance. In the control of the smelting process, the control of the alloy is also an important means.

1.3 The influence of charge ratio on materials

In the past, we have always insisted that as long as the chemical composition meets the requirements of the specification, we should be able to obtain a view that meets the standard mechanical properties of the material, but in fact this view only sees the conventional chemical composition, and ignores some alloying elements and harmful elements in it. The role of. For example, pig iron is the main source of Ti, so the amount of pig iron used will directly affect the content of Ti in the material and have a great impact on the mechanical properties of the material. Similarly, scrap steel is the source of many alloying elements, so the amount of scrap has a very direct effect on the mechanical properties of cast iron. In the early days when the electric furnace was put into use, we always used the charge ratio of the cupola furnace (pig iron: 25~35%, scrap steel: 30~35%). As a result, the mechanical properties (tensile strength) of the material were very low. When the amount of used steel has an impact on the performance of cast iron, after adjusting the amount of scrap in time, the problem is quickly solved. Therefore, scrap steel is a very important control parameter in the melting control process. Therefore, the charge ratio has a direct impact on the mechanical properties of cast iron materials and is the focus of smelting control.

1.4 The influence of trace elements on material properties

In the past, we only paid attention to the influence of the conventional five major elements on the quality of cast iron during the smelting process, while the effect of other trace elements was only a qualitative understanding, but they were rarely analyzed and discussed quantitatively. In recent years, due to the impact of casting technology Progress, smelting equipment is constantly being updated, and cupolas have been gradually replaced by electric furnaces. Although electric furnace smelting has its incomparable advantages in cupola smelting, electric furnace smelting also loses some of the advantages of cupola smelting, so the influence of some trace elements on cast iron is also reflected. Because the metallurgical reaction in the cupola is very strong, the charge is in a strong oxidizing atmosphere, most of it is oxidized, and discharged with the slag, only a small part will remain in the molten iron, so some have an adverse effect on the casting Through the metallurgical process of the cupola, the trace elements generally do not have an adverse effect on cast iron. During the smelting process of the cupola, part of the nitrogen in the coke and nitrogen (N2) in the air will dissolve into the molten iron in the form of atoms at high temperatures, making the nitrogen content in the molten iron relatively high.

According to statistics, since the electric furnace was put into operation, the waste products caused by high lead content and the scrapped molten iron because the lead content was too high to be adjusted were no less than 100 tons, and the number of unqualified products due to insufficient nitrogen content was also quite high, causing the company Great economic loss.

Based on our many years of electric furnace smelting experience and theory, I believe that the key trace elements in the electric furnace smelting process are mainly N, Pb, and Ti. The effects of these elements on gray cast iron are mainly as follows:

Lead

When the lead content in the molten iron is high (>20PPm), especially when interacting with the higher hydrogen content, it is easy to form Widmanstatten graphite in castings with thick sections. This is because the resin sand has good thermal insulation properties and the molten iron Cooling is slower in the mold, (this tendency is more obvious for thick sections), the molten iron stays in the liquid state for a longer time, and the solidification of the molten iron is closer to the solidification condition in the equilibrium state due to the action of lead and hydrogen. When this type of casting is solidified and continues to cool, the carbon in the austenite will precipitate and become secondary graphite in the solid state. Under normal circumstances, the secondary graphite only thickens the eutectic graphite flakes, which will not have a great impact on the mechanical properties. However, when the nitrogen and hydrogen content is high, the surface energy of graphite on the same fixed crystal plane of austenite will be reduced, and the secondary graphite will grow up along a certain crystal plane of austenite and extend into the metal matrix. Observe under a microscope. Many small burrs-like graphite flakes grow on the side of the flake graphite flakes, commonly known as graphite hairs, which is the reason for the formation of Widman's graphite. The aluminum in cast iron can promote the liquid iron to absorb hydrogen and increase its hydrogen content. Therefore, aluminum also has an indirect effect on the formation of Widmanstatten graphite.

When Widmanstatten graphite appears in cast iron, its mechanical properties are greatly affected, especially the strength and hardness, which can be reduced by about 50% in severe cases.

Widman's graphite has the following metallographic characteristics:

1) On the 100-fold photomicrograph, there are many small thorn-like graphite flakes attached to the coarse graphite flake, which is Widmanstatten graphite.
2) The relationship of the common crystalline graphite is connected to each other.
3) When the Widmanstatten graphite network extends into the matrix at room temperature, it becomes the fragile surface of the matrix, which will significantly reduce the mechanical properties of gray cast iron. But from the cross-sectional view, the fracture cracks still extend along the co-chip-like graphite.

Nitrogen

A proper amount of nitrogen can promote graphite nucleation, stabilize pearlite, improve the structure of gray cast iron, and improve the performance of gray cast iron.

Nitrogen has two main influences on gray cast iron. One is the influence on the shape of graphite, and the other is the influence on the matrix structure. The effect of nitrogen on graphite morphology is a very complicated process. Mainly manifested in: the influence of the adsorption layer on the graphite surface and the influence of the size of the eutectic group. Since nitrogen is almost insoluble in graphite, nitrogen is continuously adsorbed on the front of graphite growth and on both sides of graphite during the eutectic solidification process, resulting in an increase in the surrounding concentration of graphite during the precipitation process, especially when graphite extends into molten iron. At the tip, it affects the growth of graphite on the liquid-solid interface. During the eutectic growth process, there is a significant difference in the nitrogen concentration distribution at the tip and both sides of the graphite sheet. The adsorption layer of nitrogen atoms on the graphite surface can hinder the diffusion of carbon atoms to the graphite surface. When the nitrogen concentration of the graphite front is higher than that of the two sides, the growth rate of the graphite in the longitudinal direction is reduced. In contrast, the lateral growth becomes easier, and as a result, the graphite becomes shorter and thicker. At the same time, since there are always defects in the graphite growth process, a part of the nitrogen atoms are adsorbed at the defect position and cannot diffuse, and the grain boundary will be asymmetrically inclined at the front of the graphite growth, and the rest will still grow in the original direction. Graphite produces branches, and the increase of graphite branches is another reason why graphite becomes shorter. In this way, due to the refinement of the graphite structure, the splitting effect on the matrix structure is reduced, which is conducive to the improvement of the performance of cast iron.

The effect of nitrogen on the matrix structure is that it is a pearlite stabilizing element. The increase in nitrogen content reduces the eutectoid transformation temperature of cast iron. Therefore, when a certain amount of nitrogen is contained in gray cast iron, the degree of supercooling of the eutectoid transformation can be increased, thereby refining pearlite. On the other hand, because the atomic radius of nitrogen is smaller than that of carbon and iron, it can be used as interstitial atoms to dissolve in ferrite and cementite, causing its crystal lattice to be distorted. Due to the above two reasons, nitrogen can have a strengthening effect on the matrix.

Although nitrogen can improve the performance of gray cast iron, when it exceeds a certain amount, nitrogen pores and microcracks will be generated as shown in Figure 2, so the control of nitrogen should be controlled within a certain range. Generally 70-120PPm, when it exceeds 180PPm, the performance of cast iron will drop sharply.

Ti is a harmful element in cast iron. The reason is that titanium has a strong affinity with nitrogen. When the content of titanium in gray cast iron is high, it is not beneficial to the strengthening effect of nitrogen. First, it forms a TiN compound with nitrogen, which reduces In fact, it is precisely because this free nitrogen has a solid solution strengthening effect on gray cast iron. Therefore, the level of titanium content indirectly affects the performance of gray cast iron.

Melting control technology

2.1 Selection of material chemical composition

Through the above analysis, the control of chemical composition is very important in smelting technology, and it is the basis of smelting control. Therefore, a reasonable chemical composition is the basis for ensuring the performance of the material. Usually, the composition control of high-strength cast iron (tensile strength ≥300N/mm2) mainly includes etc. C, Si, Mn, P, S, Cu, Cr, Pb, N

2.3 Control technology of trace elements

In the actual process control, based on the analysis of the charge, it is confirmed that the source of lead is mainly scrap steel. Therefore, the control of lead in the raw material is mainly to control the Pb inclusions in the scrap steel, and the lead content is usually controlled below 15ppm. If the lead content in the raw molten iron is> 20 ppm, special deterioration treatment shall be carried out during the incubation treatment.

Since Ti is mainly derived from pig iron, the control of Ti is mainly to control pig iron. On the one hand, it is necessary to put forward strict requirements on the Ti content in pig iron when purchasing. Usually, the titanium content of pig iron is required to be: Ti<0.8%, and the other One aspect is to adjust the usage amount in time according to the titanium content of pig iron.

Mainly comes from recarburization materials and scrap steel, so the control of N is mainly to control recarburization materials and scrap steel. However, as mentioned above, too low and too high have a negative side to the performance of gray cast iron, so the content of N The control range is generally: 70~120ppm, but the content of N should have a reasonable match with the content of Ti. Generally, the relationship between N and Ti is: N:Ti=1:3.42, that is, 0.01% of Ti can absorb 30PPm of nitrogen. The general recommended amount of nitrogen during production is: N=0.006~0.01+Ti/3.42.

2.4 Control technology of smelting process

1) Inoculation technology

The purpose of inoculation treatment is to promote graphitization, reduce white mouth tendency, and reduce end surface sensitivity; control graphite morphology and eliminate undercooled graphite; appropriately increase the number of eutectic clusters and promote the formation of flake pearlite, so as to improve the strength performance of cast iron And other performance purposes.

The influence of molten iron temperature on inoculation and controlling the temperature of molten iron have significant influence on inoculation. Increasing the overheating temperature of molten iron within a certain range and keeping it for a certain period of time can make the undissolved graphite particles remain in the molten iron, which can be completely dissolved in the molten iron to eliminate the genetic influence of pig iron and give full play to the inoculant's inoculation effect , Improve the fertility ability of molten iron. In the process control, the overheating temperature is increased to 1500~1520℃, and the inoculation temperature is controlled at 1420~1450℃.

The particle size of the inoculant is an important indicator of the status of the inoculant and has a great influence on the inoculant effect. If the particle size is too fine, it is easy to disperse or be oxidized into the molten slag and lose its effect. If the particle size is too large, the inoculant will not melt or dissolve completely. Not only can it not fully exert its inoculation effect, but it will cause segregation, hard spots, supercooled graphite and other defects. Therefore, the particle size of the inoculant should be controlled within 2~5mm as much as possible. Ensure the incubation effect.

In the process control, the inoculation process is mainly inoculated in the incubation tank, so that the pouring of a package of castings can basically be completed before the incubation declines. But for relatively large parts and parts cast with double ladle, it cannot meet the requirements. Therefore, the late inoculation method is adopted: that is, the floating silicon inoculation is carried out in the ladle before the casting is poured (inoculation amount is 0.1%), which reduces or does not exist inoculation decline and improves the inoculation effect.

2) Alloying treatment

Alloying treatment adds a small amount of alloying elements to ordinary cast iron to improve the mechanical properties of gray cast iron. In the control of the smelting process, the addition of alloys is mainly for the parts that customers require to be quenched and the parts with relatively thick guide rails, the main alloy elements added and the amount of addition.

This ensures to a certain extent the decrease in performance due to the increase in the CE value, and for the quenched parts, the hardenability during quenching is improved. Ensure the quenching depth.

During the feeding and melting process, the feeding order of the key control at this stage is to feed the scrap steel, mechanical iron, and pig iron in the order of priority. In order to reduce the burning loss of alloying elements, the ferroalloy should be added at the end. When the cold material is completely cleared, the temperature is raised to 1450℃. That is point A. If it is lower than 1450°C, there is a risk of incomplete dissolution of the recarburizer or ferroalloy.

In paragraphs A-B, the following treatments should be done:

Temperature measurement;
Mucking slag;
Sampling and analysis of chemical composition;
Analyze conventional elements and trace elements with thermal spectrometer;
Take the triangle test piece to measure the CW value;
After adjusting the molten iron according to various test results, continue to supply power for 10 minutes and then resample and analyze. After confirming that all data is normal, continue to raise the temperature to about 1500°C, that is, point C. In the C-D section, let the molten iron stand for 5 to 10 minutes and then take a triangle test piece to test the CW value. After measuring the temperature, prepare the iron for tapping.

Triangular test piece control

For different grades, determine the white mouth (CW) control range of different triangle test blocks, and determine the quality of molten iron in combination with the composition analysis in front of the furnace.

Conclusion

The above-mentioned gray cast iron smelting technology has been successfully applied in CSMF for 8 years from 1996 to 2003. The CE of castings is controlled under the premise of 3.6~3.9, whether it is the tensile strength index or the physical hardness index (especially part of the The hardness of the guide rail of machine tool parts meets the requirements, which greatly improves the cutting performance of the casting. It has been proved that this technology is a finalized technology, and its control points are as follows:

3.1 Control of the chemical composition of materials
3.2 Determination of the ratio of charge
3.3 Control technology of trace elements
3.4 Control of inoculation treatment process
3.5 Alloying treatment
3.6 Temperature control of the smelting process
3.7 Control of triangle test piece

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