Electroslag Remelting (ESR) Purity Control of 1.2083 Mold Steel
Mold steel is an important industrial material widely used in aerospace, automotive manufacturing, electronics, toys, medical devices, and many other fields. To improve the cleanliness, wear resistance, and impact resistance of this steel to meet user requirements, research was conducted on the electroslag remelting process control for this series of steels. This article focuses on the causes and metallurgical mechanisms of excessive Class C inclusions during steelmaking and electroslag remelting processes. It studied the composition of the electroslag pre-melt slag and the electroslag remelting process technology (deoxidizer usage), and by optimizing the process parameters, it reduced the problem of excessive Class C inclusions, with the produced products meeting user requirements.
1 Industrial Trial of 1.2083 Steel
1.1 Industrial Trial Process The production process flow for 1.2083 steel is: initial refining furnace (100-ton converter, 100-ton electric furnace, 50-ton alloy melting furnace) → 50-ton AOD refining furnace → 50-ton LF refining furnace → continuous casting of 180 square billets → electroslag remelting of 5-ton ingots → slow cooling annealing of ingots → heating and rolling of ingots → slow cooling annealing of rolled products → warehousing. The rolled product specifications are 30~120 mm thick × 300~610 mm wide. Electroslag remelting involves welding 180 square continuous cast billets into a “田” shape, using a crucible with a Φ660 mm ingot shape. Three industrial electroslag remelting trials were conducted using our plant’s constant power 5-ton non-gas protective double-arm electroslag furnace, with various process parameters as shown in Table 2.
1.2 Melting Composition, Inclusions, and Slag System Components of the Steel
2 Comparison of Testing Data Before and After Different ESR Processes
The composition of the ingots before and after electroslag remelting is shown in Table 3, with corresponding inclusion ratings presented in Table 5. Using the original Process A, over 30 electroslag ingots produced during the same period resulted in rolled finished products that, upon high-power inspection (inclusion detection according to GB/T10561-2023-A method), showed that 15 batches of finished products had Class C fine or coarse inclusions exceeding the standard by 0.5 to 1.5 grades, reaching 1.5 to 2.5 grades. The inclusions in the metallic raw materials (continuous cast billets) were relatively pure, with their inclusion control level higher than those of the electroslag finished products using Process A; no batch of Class C (silicate inclusions) was found in the metallic raw materials. After analyzing four batches of original continuous cast billets, the [O] content was controlled at ≤20×10^-6. Several electroslag finished products using Process A were analyzed, showing [O] content controlled between 50 to 60×10^-6, indicating a significant increase in [O] during the electroslag remelting process, which influenced the generation of inclusions. See Figures 1(a) and 1(b).
Using Process B for electroslag remelting resulted in lower oxygen content compared to Process A and better inclusion control. Using Process C further reduced the oxygen content of the electroslag ingots, with the inclusion ratings of the rolled products meeting the product requirements.
3 Analysis of Results from Different ESR Processes
Metallurgical analysis of the experimental results indicated that the original electrode billets were relatively pure, with oxygen content around 20×10^-6. Using Process A increased the oxygen content to above 50×10^-6. Using Process B, increasing CaO in the slag reduced the oxygen content to levels around 36×10^-6. Using Process C, increasing the CaO content in the slag and adding deoxidizers at a rate of 1 kg per ton of steel controlled the oxygen content to 25×10^-6.
Using the 3:7 pre-melt slag with Process A inevitably contains about 1% SiO2 impurities, and ordinary electroslag furnaces contain a large amount of oxygen. Surface oxidation occurs when metallic electrodes are exposed to high temperatures (4Fe + 3O2 = 2Fe2O3). A large amount of Fe2O3 from the surface of the metallic electrodes enters the molten slag, increasing the oxygen potential of the slag, i.e., FeO levels in the slag rise sharply. According to the reaction [Si] + 2(FeO) = SiO2 + 2[Fe], a large amount of SiO2 is generated in the steel, some of which remains in the electroslag ingot, causing the Class C inclusions in the rolled products to exceed standards. Some of the SiO2 enters the slag, significantly increasing the activity of SiO2 in the slag. According to the reaction [Si] + 2[O] = (SiO2), when the SiO2 level in the slag rises significantly, the equilibrium oxygen in the steel also increases, resulting in the oxygen content in the electroslag ingot rising to 55×10^-6.
Using Process B increased the CaO in the slag, allowing it to combine with SiO2 in the slag to form CaO·SiO2, thereby reducing the activity of SiO2. According to the reaction [Si] + 2[O] = (SiO2), when the SiO2 level in the slag rises, due to part of the SiO2 combining with CaO to form CaO·SiO2, the activity of SiO2 can be reduced, thus the equilibrium oxygen in the corresponding steel is lower than that of the 3:7 pre-melt slag used in Process A, achieving 36×10^-6, with the corresponding inclusion levels also improved.
Using the aluminum powder deoxidizer process (1 kg per ton of steel) with Process C allows the aluminum powder entering the steel to react as 2Al + 3FeO = Al2O3 + 3Fe, reducing the content of unstable oxide FeO and suppressing the reaction [Si] + 2(FeO) = SiO2 + 2[Fe]. The increase in the activity of SiO2 in the slag becomes extremely slow. According to the reaction [Si] + 2[O] = (SiO2), when there is little change in the SiO2 level in the slag, the equilibrium oxygen in the corresponding steel is also low, so the oxygen content in the electroslag ingot is further controlled to 25×10^-6 levels, with the corresponding inclusion levels further reduced to meet product requirements. Comprehensive evaluation is shown in Table 6 and Figure 2.
4 Improvement Measures During the ESR Process
With the continuous development of high-quality special steels through electroslag remelting, the requirements for products are becoming increasingly stringent. Preventing the oxidation of metallic electrodes and reducing the oxygen content in electroslag ingots will become important trends in the future. Using atmosphere protection electroslag remelting technology to prevent the oxidation of metallic electrodes is an important method to improve the quality of electroslag products. Therefore, our company introduced atmosphere protection electroslag modification technology from Suzhou University, capable of reducing the oxygen partial pressure inside the electroslag furnace to the level of 1×10^-6, comparable to gas-protected electroslag furnaces, as shown in Figure 3, improving the overall competitiveness of our company’s products.
In summary, improving the basicity of pre-melt slag, adding deoxidizers, and using atmosphere protection electroslag modification technology are effective means of reducing inclusions. Through the research conducted in this study, five electroslag steel ingots were smelted and rolled, with samples taken from the head and tail of the finished products corresponding to the electroslag ingots, all Class C fine and coarse inclusions rated at 0 grade, meeting the process requirements.
5 Conclusion
(1) Excessive Class C inclusions in 1.2083 electroslag steel are caused by the surface oxidation of metallic electrodes during remelting, where the scale enters the slag, increasing the oxygen potential of the slag. Silicon in the molten metal is continuously oxidized by FeO in the slag, increasing the SiO2 content and activity in the slag, raising the equilibrium oxygen, and increasing the inclusion content in the electroslag ingot.
(2) Research and optimization of the process and slag system ratio, increasing the CaO content to 10%, and adding strong deoxidizers during smelting, with 1 kg of deoxidizer added every hour during remelting, can effectively inhibit the SiO2 content in the slag and reduce the level of Class C inclusions.
(3) Through analysis of the causes affecting cleanliness and process optimization, the products produced meet user requirements.
