To improve the production efficiency, reduce process costs and achieve mass production capacity of ultra-high strength steels above 2000 MPa grade, industrial trials were conducted to replace the VIM-ESR process with the EAF→LF→VD process. The test results show that the total mass fraction of oxygen, nitrogen and hydrogen in the steel is less than \(50×10^{-6}\); the non-metallic inclusions of Class A, B and C are evaluated as Grade 0, and Class D inclusions are no higher than Grade 1.0. After quenching and low-temperature tempering, the finished steel plates have a tensile strength exceeding 2100 MPa and an elongation greater than 7%, with performance equivalent to that produced by the VIM-ESR process. The new process reduces production costs by more than 44% and significantly improves production efficiency, making it suitable for large-scale production of low alloy ultra-high strength steels.
Keywords: Ultra-high strength steel; Electric arc furnace; Refining; Process trial
Ultra-high strength steels, with strength exceeding 1500 MPa and certain toughness, are widely used in aircraft landing gears, rocket engine casings, missile shells, large ship structures and protective components for weapon systems. As equipment manufacturing develops toward high performance and low cost—especially for weight reduction to improve mobility and load capacity—the application of ultra-high strength steels in load-bearing and impact-resistant parts of machinery, vehicles and weapon systems is expanding rapidly.
Currently, ultra-high strength steels above 2000 MPa grade are mainly produced by VIM-ESR (Vacuum Induction Melting-Electroslag Remelting) or VIM-VAR (Vacuum Induction Melting-Vacuum Arc Remelting) duplex processes. However, these processes have extremely high requirements for raw material quality, low production efficiency and high process costs, making it difficult to achieve mass production and restricting the wider application of ultra-high strength steels.
In this trial, initial molten steel was smelted in an industrial-scale electric arc furnace. Through enhanced LF (Ladle Furnace) refining, VD (Vacuum Degassing) treatment and argon-protected casting, high-purity molten steel was obtained. The produced steel meets design requirements and has performance equivalent to that from the vacuum induction melting plus electroslag remelting process, while significantly reducing costs and improving production efficiency.
The test steel is grade 50Si2Ni2CrMoV, with control ranges for main elements and gas contents shown in Table 1.
Table 1 Chemical compositions of test steel (\(w_B\))/%
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The mechanical properties of finished steel plates after quenching and tempering shall meet the requirements specified in Table 2.
Table 2 Mechanical properties of finished steel plate after quenching and tempering
Scrap steel was charged into a 20 t ultra-high power electric arc furnace (EAF) for primary melting. The tapped molten steel was transferred to a 25 t LF for refining, followed by VD treatment. The steel was cast into ingots under argon protection, heated and rolled into 15 mm thick plates by a 3300 mm rolling mill.
Since the steel achieves strength exceeding 2100 MPa after final heat treatment while requiring sufficient toughness, the molten steel must be extremely pure, with phosphorus, sulfur and residual harmful elements controlled to the lowest possible levels. The smelting process focuses on degassing, inclusion removal and deoxidation. The test adopted high-temperature oxidation and high decarburization in EAF for degassing and inclusion removal, low-temperature dephosphorization, full submerged arc operation in LF to prevent air suction, enhanced deoxidation, precise control of carbon and alloy elements, and high vacuum with strong stirring in VD to enhance degassing and refining effects.
EAF Stage: First, high-quality scrap steel was used to avoid introducing harmful elements such as As and Sn that contaminate the molten steel. Second, the initial carbon mass fraction must be greater than 0.60% (or even higher) to ensure sufficient boiling of the molten steel through carbon-oxygen reaction for degassing and inclusion removal, with oxidation temperature exceeding 1570℃. At the end of oxidation, phosphorus mass fraction reached 0.003% and carbon mass fraction reached 0.006%. The oxidation slag must be completely removed to prevent phosphorus reversion. After slag removal, Fe-Si was added for pre-deoxidation, followed by slag formation and pre-reduction with carbon powder and C-Si powder.
LF Stage: After entering the LF, aluminum was added for pre-deoxidation, and C-Si powder slag was formed for white slag refining. Utilizing the good reducing atmosphere and argon stirring in the LF, full refining was carried out for further deep deoxidation, desulfurization and inclusion removal. The entire refining process used submerged arc heating to avoid air suction and maintain a reducing atmosphere. Argon was blown from the bottom of the ladle to stir the molten steel, increasing the contact area between the molten steel and alkaline slag and accelerating steel-slag reactions, with white slag maintained for more than 20 minutes.
VD Process: During VD treatment, the vacuum degree reached below 67 Pa (maximum 25 Pa) with a total time of 25 minutes. The argon flow rate was increased and strong stirring was applied to further reduce hydrogen, nitrogen, oxygen and inclusions in the molten steel.
To minimize or avoid atmospheric contamination of the molten steel during casting, an argon protection device was installed at the bottom of the ladle, and argon was supplied throughout the casting process. The steel flow system used high-strength, high-temperature resistant and erosion-resistant refractory bricks to reduce secondary air suction, oxidation and contamination during casting.
To compare the finished product performance and microstructure of steels produced by the two processes, samples for chemical composition and gas analysis were taken after VD treatment, while samples for mechanical properties and metallographic examination were taken from the finished steel plates. According to GB 2975, three samples each were taken from finished plates produced by the electric arc furnace process and the vacuum induction melting plus electroslag remelting process. After heat treatment, tensile and hardness tests were conducted, and quenched and tempered microstructure analysis was performed. Two samples were randomly taken from the steel plates for inclusion rating according to GB 10561.
The chemical composition of the steel meets the design and control requirements. The comparison of gas and harmful impurity contents is shown in Table 3. It can be seen that the total mass fraction of oxygen, nitrogen and hydrogen in the steel smelted by the EAF→LF→VD process is \(30.3×10^{-6}\), compared with \(34.3×10^{-6}\) for the original process, both reaching very high levels. The contents of gaseous and harmful impurities P and S are comparable to those of the VIM-ESR process, and As and Sn meet the control requirements but are higher than those of the VIM-ESR process.
The VIM process requires purer charge materials and maintains a vacuum degree generally below 1 Pa during smelting, allowing As and Sn to volatilize under high vacuum. In contrast, the electric arc furnace process uses relatively lower-quality charge materials and operates at atmospheric pressure. With a VD vacuum degree of 67 Pa (maximum 25 Pa), As and Sn are difficult to remove. While the VIM process has advantages in reducing gases, P, S, As and Sn (with oxygen and nitrogen mass fractions around \(15×10^{-6}\)), the electroslag remelting process under non-vacuum and non-protected conditions causes the molten steel to absorb some hydrogen and nitrogen, increasing their contents in the final steel.
Table 3 Comparison of the contents of gas and impurity elements \(w_B\)/%
The inclusion detection and rating comparison is shown in Table 4. The EAF→LF→VD smelting process was well controlled with sufficient inclusion removal. Almost no Class A, B or C inclusions were observed in the field of view, while Class D inclusions were rated as Grade 0.5. The test results are superior to those of the VIM-ESR process, reflecting high steel purity and providing a solid guarantee for the performance of the steel plates.
Table 4 Comparison of non-metallic inclusions in steels
After the same heat treatment regime (900℃×20 min oil quenching, 270℃×2 h air cooling), the steel plate samples produced by both processes exhibited fine tempered martensite structures with identical morphology.
The quenched and tempered samples were tested on a tensile testing machine and Brinell hardness tester, with results shown in Table 5. The finished steel plates produced by both processes have comparable mechanical properties after heat treatment, with strength, toughness and hardness all meeting the product technical standard requirements.
Table 5 Comparison of mechanical properties of steel plate samples after quenching and low tempering
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In practical applications, 2100 MPa grade low alloy ultra-high strength steels are all subjected to quenching plus low-temperature tempering. After this heat treatment, the steel obtains lath martensite with high dislocation density and fine, dispersed coherent precipitated ε-carbides, which hinder dislocation movement and give the steel very high strength. Compared with steel smelted by the VIM-ESR duplex process, the test steel produced by EAF→LF→VD has similar carbon content, alloy element content, and identical pressure processing and heat treatment processes. Therefore, it can be confirmed that the microstructural parameters such as grain size, quenched and tempered structure, and carbide type, size, distribution and dispersion degree are consistent. Under these conditions, the comprehensive performance is determined by steel purity, i.e., the content of gases and harmful elements, and the content, distribution and morphology of inclusions.
Numerous studies on steel performance have shown that inclusions directly affect the strength and toughness of steel. In particular, the plasticity and toughness of ultra-high strength steels are more sensitive to inclusions, especially large-particle inclusions which are catastrophic. Therefore, it is essential to minimize the contents of oxygen, nitrogen and hydrogen, and reduce harmful elements such as sulfur and phosphorus to purify the molten steel. Extremely low inclusion content and fine grain size are necessary to give ultra-high strength steels sufficient toughness for practical applications.
In the EAF→LF→VD test process, the high-temperature oxidation and high decarburization in the EAF stage ensured sufficient boiling of the molten steel for degassing, while dephosphorizing to 0.003%. In the LF stage, full submerged arc operation and bottom argon blowing throughout the process enhanced steel-slag reactions and full refining, playing a good role in desulfurization, deoxidation, inclusion removal and degassing. Calcium treatment was applied to the molten steel before the end of the LF process to improve inclusion morphology and distribution. Combined with the subsequent VD vacuum treatment, the control of gas contents (O, N, H) and P, S in the molten steel reached the level of high-purity steel. No Class A, B or C inclusions were found during evaluation, and Class D inclusions were rated as Grade 0.5. The performance test results of the finished steel plates meet the specified requirements and are equivalent to those produced by the VIM-ESR process.
Processes such as vacuum induction melting, induction (vacuum induction) plus electroslag remelting, vacuum arc remelting, and their combined duplex or triplex processes can all effectively achieve high or ultra-high cleanliness of molten steel. However, these methods have extremely high requirements for the purity of incoming materials: ordinary scrap steel cannot be used, and steel materials must be pure iron. Moreover, pure iron and alloy materials all require pretreatment to control purity, resulting in charge costs far exceeding those of the electric arc furnace process. Process costs are also higher than those of the electric arc furnace smelting process. The cost comparison between EAF→LF→VD and VIM-ESR is shown in Table 6, with smelting cost reduced by 44%.
Table 6 Comparison of the cost of smelting processes
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Note: Charge costs vary with prices of alloys, scrap steel and pure iron; process costs vary with furnace type and operating conditions.
Special smelting furnaces generally have small capacities, usually ranging from 500 kg to 3 t. In China's special steel industry, there are only 5 vacuum induction furnaces with capacities of 5~12 t, which significantly limits production efficiency. In contrast, the electric arc furnace plus refining process has low requirements for charge materials, low process costs, and a single furnace output of more than 20 t (even achievable in 50 t capacity furnaces). The EAF→LF→VD process has very obvious advantages in reducing production costs and improving production efficiency, and can be extended to the production of other grades of low alloy ultra-high strength steels.
Under the conditions of using high-quality charge materials, enhanced refining and protected casting, the 2100 MPa grade ultra-high strength steel produced by the EAF→LF→VD process achieves a total mass fraction of oxygen, nitrogen and hydrogen below \(50×10^{-6}\), Class A, B and C inclusions rated as Grade 0, and Class D inclusions rated below Grade 1.0. The steel performance is equivalent to that produced by the VIM-ESR process.
The adoption of the EAF→LF→VD process reduces the smelting cost of 2100 MPa grade ultra-high strength steel by 44% and significantly improves production efficiency, enabling mass production.