The smelting process of high-quality steel castings by steel casting manufacturers

With the acceleration of social development in our country, the production technology and level of various industries have also been correspondingly improved. Steel casting manufacturers continue to develop and produce steel castings, and the proportion of production and use of steel castings has also increased. In order to establish a foothold in fierce market competition, steel casting manufacturers must improve their smelting technology and steel casting manufacturing technology. To achieve high-quality production in the manufacturing of steel castings, it is necessary to conduct in-depth research on the characteristics of cast steel materials, explore the smelting process of cast steel, scientifically choose production methods, innovate production processes, improve the production quality of cast steel parts, fully leverage the characteristics of high temperature resistance, strong hardness, and strong compressive strength of cast steel parts, and make contributions to the long-term stable development of China’s cast steel industry.

1. Basic concepts of cast steel

Molten iron is used to produce cast iron. When the strength of steel castings is high, the supply of iron materials used for casting production is insufficient, and the production of cast iron cannot meet the actual production requirements of parts, casting steel production technology can be applied according to actual needs. Compared with the iron liquid used to make iron parts, the fluidity of the steel liquid used to make steel parts is lower. During casting, special attention should be paid to the thickness and shape of the iron parts, and the upper limit of mineral element content should be controlled to effectively improve the fluidity of the steel liquid. Steel castings can be divided into four categories based on their types and applications, namely ordinary steel castings, welding steel castings, stainless steel, and heat-resistant steel castings.

2. Smelting technology for high-quality steel castings

With the increasing variety of cast steel products and higher quality requirements, it is necessary to control the gas content and non-metallic inclusions in the steel liquid, as well as analyze metallurgical defects such as porosity, hot cracks, and brittle cracks in cast steel. These two aspects are important factors affecting the smelting quality and are also the key for casting steel manufacturers to improve product quality. According to research findings, selecting a suitable smelting furnace and adopting vacuum smelting technology can effectively reduce the gas content and non-metallic slag content during the smelting process. On this basis, exploring the smelting technology of cast steel under vacuum free conditions can effectively improve the quality of cast steel. The requirements for the smelting process of steel castings in actual production are very high. In the preparation stage of production, the most scientific smelting process should be selected according to the characteristics of the use of steel castings for the production and manufacturing of parts. This is the foundation for the successful production of steel castings. In the actual production stage of steel castings, suitable smelting processes and methods should be selected according to the production operation requirements, especially strict control of the use of raw materials and scrap steel, and records of the production process and raw materials should be kept to facilitate timely inspection in case of problems.

2.1 Electroslag Casting Technology

Electric slag melt can be used for the production of high comprehensive performance blanks due to its dense structure, high purity, and uniform composition. The electric slag remelting equipment and technology in countries such as the United States, Germany, and Sweden are relatively mature and widely used in aerospace, military, nuclear power, and other fields. In the theories of thermal equilibrium calculation and slag system development, there have been many original studies in China. Shenyang Foundry has applied electroslag casting technology to the production of water turbine guide vanes. Its low impurity element content, dense structure, and high material utilization rate have significantly improved its plasticity, toughness, fatigue resistance, cavitation resistance, and other properties. The new technology of electroslag remelting metallurgy, represented by electroslag continuous casting, controllable atmosphere, and liquid electroslag casting, is closely integrated with the steel smelting process and has broad application prospects.

2.2 Lost foam casting technology

The lost foam casting technology is widely used in complex shapes such as tubes, boxes, and cylinder bodies due to its characteristics of near zero allowance and precise forming. It is mostly used for wear-resistant, heat-resistant, and corrosion-resistant steel castings. On this basis, a mathematical model of gas diffusion, heat transfer and melting between molten metal liquid and foam mold in molten state is proposed, which provides a theoretical basis for mold filling and solidification. In order to solve the carbonization problem in the casting process, while reducing the carbon content in the foam model, some new lost foam casting technologies are gradually popularized, such as oxygen rich combustion technology, negative pressure combustion shell casting, carbon removal method and filmless seal. Due to the growing demand for medium and large sized cast steel products with high comprehensive performance, this technology has entered a stage of rapid development, and will carry out in-depth research in advanced processes, new foam mold materials, material composition design, synthetic coatings, waste gas environmental purification, intelligent manufacturing and other fields.

3. Smelting process control of high-quality steel castings

LF refining occurs after deoxidation of molten steel. Baosteel undergoes deoxidation during the EAF steelmaking process, while BOF undergoes deoxidation alloying in RH. When the molten steel enters the LF station, its dissolved oxygen mass fraction has reached 3.0 × 10 ^ -5. During the LF refining process, diffusion deoxidizers are added to the slag to rapidly reduce oxygen in the slag and steel, resulting in a dissolved w [O] of less than 1.0 × 0-3 in the molten steel and a w (FeO) of 0.1% to 0.5% in the slag. If barium based deoxidizers are used, the oxygen w [O] in the steel can reach (2-3) × 10-4, and the w (FeO) in the slag is less than 0.04%. The LF refining endpoint is a typical chemical composition of the slag. The experimental results indicate that the process has good reduction and dephosphorization conditions in a low fluidized bed. In addition, the LF refining process is high-temperature, and the effect of reducing phosphorus removal is better than that of oxidizing phosphorus removal; The use of Al Ca Ca alloy can avoid excessive calcium vaporization and loss; The use of Mg based refractory materials can minimize the corrosion of reducing dephosphorization on the furnace lining. Therefore, the reduction dephosphorization in LF refined steel is feasible and can also reduce the sulfur content in the steel. The results show that the oxidation rate of the steel is (1~13) × 10 ^ -6, with an average of 5.27 × 10 ^ -6. Without foam slag method, the w (N) in the steel is (0~20) × 10 ^ -6, with an average of 6.43 × 10-6. For steel grades with strict control over nitrogen content, 3-4 batches of foaming agents should be added according to the required smelting time, and aluminum should not be added during the smelting process.

4. Quality inspection of steel castings

The casting process of large steel castings is complex, with long production cycles and multiple processes, which inevitably leads to some casting defects. These defects will have varying degrees of impact on the appearance and internal quality of castings, thereby having a certain impact on the service performance of castings. To obtain high-quality large-sized steel castings, standardized testing must be carried out. In the quality inspection process of casting products, dimensional testing, mechanical performance testing, and non-destructive testing are important means to achieve production process optimization and quality control.

4.1 Dimensional determination of large diameter cast steel parts

At present, the commonly used measurement methods in large steel castings are manual measurement and 3D marking instruments. However, existing measurement methods have problems such as low accuracy, low efficiency, and easy generation of manual and calculation errors, which seriously restrict the production efficiency of steel castings. Meanwhile, with the increasingly complex size and shape design of large and medium-sized steel castings, higher quality requirements have been put forward, and traditional size testing can no longer meet the size testing requirements of large and medium-sized steel castings. In recent years, China has made great progress in the size testing of large steel castings, and has applied optical and intelligent spatial coordinate technology to actual production. Due to its advantages of high accuracy, high efficiency, and the ability to automatically fit and calculate data, this method is gradually replacing traditional methods.

4.2 Non destructive testing

The commonly used inspection methods in the casting process include magnetic particle inspection, ultrasonic inspection, X-ray inspection, etc. The use of magnetic particle method for non-destructive testing of specimens can only perform non-destructive testing on surface defects of specimens, and cannot perform non-destructive testing on internal defects of specimens. Ultrasonic testing has high requirements for the surface smoothness of the tested object, and defects cannot be directly revealed. Low energy X-ray photography also has its shortcomings, such as poor penetration, low detection efficiency, and cumbersome management of film reading and documents. The high-energy X-ray digital radiation imaging system (DR) for steel castings, which has been applied in production, has advantages such as strong penetration ability, fast detection speed, intuitive and convenient judgment, and convenient digital archive management. It can achieve fast, real-time, and online detection of internal defects in steel castings, but there are defects such as image overlap, making it difficult to achieve higher accuracy detection and measurement. By combining industrial CT with DR technology, the above defects can be solved. The high-energy density industrial CT/DR detection technology based on this technology has broad application prospects in achieving rapid and high-precision detection of the inner surface quality of steel castings.

4.3 Control of phosphorus

Phosphorus is an important harmful element in steel, which has a significant impact on the ductility, low-temperature impact and other properties of steel, making it prone to cold embrittlement. Unlike desulfurization, phosphorus removal needs to be carried out in a low temperature, high alkali, and high oxidation environment. The phosphorus removal process is divided into three stages: pre-treatment, converter, and secondary refining. In the pre-treatment stage of molten iron, although the amount of slag generated by the adopted process is very small, desulfurization must be carried out first, which will cause certain temperature loss. At the same time, the proportion of scrap steel in converter smelting cannot be too high. Due to the good stirring conditions in the converter, steel slag is easily separated, but it also requires high temperature, slag amount, and oxygen level. During the secondary refining process, the amount of slag is very small, but due to the need to heat the molten steel, the slag needs to be removed before deoxidation, resulting in a certain temperature loss. Dephosphorization is required to be carried out in an environment with high oxygen, high alkalinity slag, low temperature, and strong stirring. Practice has shown that although the dephosphorization effect of the converter smelting process is good, it cannot be achieved in one slag production and phosphorus cannot be reduced to the standard requirements. Therefore, when producing ultra-low phosphorus steel, dephosphorization needs to be carried out in two steps: first, hot metal pretreatment and converter blowing, followed by initial dephosphorization and deep dephosphorization in the converter.

4.4 Online hydrogen determination

The hydrogen dissolved in steel is the fundamental cause of defects such as shrinkage, white spots, cracking, and various bubbles. The presence of hydrogen reduces the strength limit, cross-sectional shrinkage, elongation, impact toughness, and other properties of molten steel, especially the latter two. Hydrogen in steel can seriously affect its performance. Currently, the w (H) of clean steel generally does not exceed 2 x 10 ^ -6. The principle of online hydrogenation is to use a circulation pump to inject nitrogen into the molten steel and make it flow in the steel. At the same time, the hydrogen gas in it is dispersed into the circulation gas and sucked into the ventilation pipe for cycling until the nitrogen and hydrogen reach saturation. On this basis, the hydrogen partial pressure was analyzed to determine the hydrogen content in the molten steel. Usually, to determine hydrogen gas, it requires sampling and submission to the laboratory, which takes a long time. Therefore, it cannot be used as a commonly used measurement method in actual production. The online hydrogen measurement system can complete the determination of hydrogen content in molten steel within 40-70 seconds, providing reliable hydrogen content data for steelmaking personnel and having certain guiding significance for actual production.

4.5 Removal of endogenous inclusions

One of the main methods for removing inclusions in domestic and foreign converter smelting processes is blowing oxygen for decarburization. The gases such as CO and CO2 generated during blowing oxygen for decarburization will bring inclusions into the steel, causing them to suspend and thus achieve purification of the steel. Based on years of production experience, if the decarburization amount exceeds 0.4%, 80% of the inclusions in the steel liquid can be removed. However, in the process of blowing oxygen for decarbonization, oxygen cannot react 100% with carbon and needs to react with other elements, such as Fe, Si, Mn, Cr, etc., to generate oxides such as FeO, SiO2, MnO, Cr2O3, and their composite oxides, which are the main sources of endogenous oxide inclusions. After the completion of oxygen blowing decarburization, the steel liquid will undergo reduction, and the reduction products usually form oxides such as SiO2 and Al2O3 or their composite oxides. These reduction products are the main components of the reduction products of endogenous inclusions.

For high-purity steel, it is necessary to strengthen the control of oxygen during smelting. The oxygen content can be reduced through top and bottom combined blowing converter, and then slag free tapping, aluminum protection slag, and crystallizer protection slag can be used to deoxygenate the steel liquid and increase the oxygen level of the slag, which can effectively prevent secondary oxidation. Secondly, in the smelting process of high-purity steel, it is necessary to increase the control of sulfur content, pay attention to the form of sulfur in the steel, mainly sulfides. This state can easily cause cracking of the steel billet and affect the corrosion resistance of the steel, and have a certain impact on the toughness of the steel. Scientific control of sulfur content is necessary. Firstly, it is necessary to smelt it, desulfurize the molten iron during the smelting process, and then protect the oxygen content of the metal and slag; Secondly, in the smelting process, attention should be paid to desulfurization, especially for the secondary refining of molten steel. Synthetic slag desulfurization can usually be used, or desulfurization can be carried out through powder spraying, heating slag making, and argon blowing and stirring. It is necessary to maintain a low content of oxygen in the slag and molten metal. In addition, desulfurization of the secondary and converter molten steel is also required during the smelting process.

5. Conclusion

In the current production environment, if vacuum smelting equipment is not used, in order to improve the smelting process, it is necessary for staff to strengthen technical research and strictly follow the process and production operating procedures. When selecting the smelting process, it is necessary to scientifically and reasonably choose the smelting process based on the shape and organizational characteristics of the steel castings. Professional management personnel should be designated to strictly supervise each link in production and manufacturing, so as to achieve the production and manufacturing of high-quality steel castings.

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