Introduction
Stainless steel is a crucial material in various industries due to its corrosion resistance, strength, and aesthetic appeal. The smelting processes for producing stainless steel have evolved significantly over the years to enhance efficiency, reduce costs, and improve product quality. This article explores the main smelting processes currently used in stainless steel production: the one-step, two-step, and three-step processes, as well as the innovative RKEF+AOD combined process.
One-Step Smelting Process
The early one-step smelting process for stainless steel involved melting scrap steel, decarburization, reduction, and refining all within a single electric furnace, producing stainless steel in one go. However, due to the long smelting cycle, low operational efficiency, and high production costs, this method has been gradually phased out. With advancements in external refining technologies and the widespread use of AOD (Argon Oxygen Decarburization) furnaces, a new one-step smelting process has emerged. This modern method often uses low-phosphorus or dephosphorized molten iron instead of scrap steel as the raw material, which is then refined in an AOD furnace.
The new one-step smelting process offers several advantages over the old method:
- Reduced investment costs.
- Lower production costs.
- Decreased material costs and energy consumption, resulting in purer steel.
- Adaptability to the current scrap steel market due to lower scrap steel usage.
- Economic efficiency, especially for producing 400 series stainless steel.
However, this process has specific requirements for raw materials and product schemes. The AOD furnace input iron must have a phosphorus content below 0.03%, necessitating an additional dephosphorization step. Furthermore, the high-carbon alloys added in the AOD furnace can affect the heat balance, making it unsuitable for complex stainless steel varieties with high alloy content.
The new one-step process is widely used for producing 400 series stainless steel in China, where scrap steel resources are scarce and nickel is expensive. This method’s growing adoption is driven by the increasing demand for 400 series stainless steel in daily life and industrial applications.
Two-Step Smelting Process
The two-step smelting process involves using an electric arc furnace (EAF) followed by an AOD or VOD (Vacuum Oxygen Decarburization) furnace. This method accounts for about 70% of the world’s stainless steel production. The EAF melts scrap steel and alloy raw materials to produce a pre-melted stainless steel, which is then refined in the AOD furnace to produce high-quality stainless steel.
If the raw materials contain high phosphorus levels, an additional dephosphorization step between the EAF and AOD furnace is required. Common pre-melt dephosphorization methods include converter dephosphorization and top-spraying dephosphorization in ladles.
The two-step process is versatile and widely used for producing various stainless steel grades. Its advantages include less stringent raw material requirements, a shorter production cycle compared to the one-step process, and the flexibility to produce 95% of stainless steel grades, excluding ultra-low carbon and nitrogen grades.
However, this process requires attention to three aspects:
- Despite improvements, the consumption of gases like argon remains higher than in one-step and three-step processes.
- High consumption of ferrosilicon due to the need for oxygen reduction in the AOD furnace.
- The inability to produce ultra-low carbon and nitrogen stainless steels, and higher gas content in the final product.
Three-Step Smelting Process
The three-step smelting process involves an initial smelting furnace, a secondary refining furnace (AOD or duplex furnace), and a vacuum refining unit. This advanced method is suitable for specialized stainless steel manufacturers and integrated steel companies, producing high-quality stainless steel with low nitrogen, hydrogen, oxygen, and inclusions.
The advantages of the three-step process include:
- Clear division of labor in each step, optimizing production and operation.
- High product quality and a wide range of producible grades.
- Flexibility in raw material selection, including the use of molten iron.
However, the three-step process requires significant investment and higher production costs due to the additional process stages and complex vacuum systems.
Globally, the two-step process (EAF+AOD or EAF+VOD) is the most widely used, accounting for about 70% of production, while the three-step process accounts for approximately 20%. The one-step process is increasingly adopted for its cost efficiency and suitability for producing 400 series stainless steel, especially with the use of low-phosphorus molten iron.
RKEF+AOD Combined Process
In 2010, Tsingshan Group pioneered the RKEF (Rotary Kiln-Electric Furnace) production line in Fujian Dingxin, China. This method, which uses nickel-rich iron ore to produce nickel pig iron (NPI), offers significant advantages over traditional methods. The fully enclosed production process minimizes energy loss and dust emissions, reducing power consumption by 1500 kWh per ton of iron and lowering dust emissions by 80%.
Tsingshan Group integrated NPI production with stainless steel refining, creating the RKEF+AOD combined process. This innovative approach, which received a national invention patent in 2014, enables continuous and integrated stainless steel production from laterite nickel ore to hot-rolled stainless steel strips. The process significantly enhances production efficiency, reduces raw material consumption, and minimizes environmental impact, marking a historic breakthrough in stainless steel manufacturing.
Conclusion
The evolution of stainless steel smelting processes reflects the industry’s ongoing efforts to improve efficiency, reduce costs, and enhance product quality. The one-step, two-step, and three-step processes each offer unique advantages and are suited to different production needs and raw material availabilities. The innovative RKEF+AOD combined process, pioneered by Tsingshan Group, represents a significant leap forward in stainless steel production technology. As the industry continues to innovate, the choice of smelting process will play a critical role in meeting the demands of the global stainless steel market.