Baotou Steel (Baotou Iron & Steel Group), located in Inner Mongolia, China, is uniquely positioned for this task. Its proximity to the massive Bayan Obo mining area, the world’s largest source of rare earth elements (REEs), gives it a strategic advantage in producing “Rare Earth Steel.” The addition of rare earths is a sophisticated metallurgical process aimed at significantly enhancing the purity, toughness, and fatigue life of the rails.
Step 1: Steelmaking – The Foundation
The process begins with creating high-purity steel
– Raw Materials: Baotou Steel uses iron ore and coal, but notably, the iron ore from Bayan Obo contains rare earth elements. However, for controlled addition, the primary rare earths are added later in a more precise form.
– Basic Oxygen Furnace (BOF): Iron ore is smelted in a blast furnace and then converted into steel in a BOF. Here, oxygen is blown through the molten iron to reduce carbon and impurity levels. The goal is to achieve a clean, low-impurity base steel.
Step 2: Secondary Refining – Achieving Ultra-High Purity
This is a critical stage for high-speed rail steel. The molten steel from the BOF is transferred for secondary treatment.
– Ladle Furnace (LF): The steel is reheated and stirred in an arc furnace to achieve precise temperature control and chemical composition.
– Vacuum Degassing (e.g., RH Degasser): The steel is placed under a vacuum. This process effectively removes dissolved gases, particularly Hydrogen. Hydrogen is extremely detrimental as it causes internal cracking (flakes) under stress, a critical failure point for rails. Achieving an ultra-low hydrogen content is non-negotiable for HSR rails.
Step 3: The Crucial Step – Rare Earth Addition
This is where Baotou Steel’s unique process comes into play. The rare earths, typically a mixture of Cerium (Ce) and Lanthanum (La), are added after the de-gassing process is complete but before casting.
Form of Addition: The rare earths are not added in pure metal form. They are added as a Rare Earth Silicon Iron (RE-Si-Fe) alloy wire. This wire is fed directly into the ladle filled with the refined molten steel.
Why this method? Feeding a cored wire ensures the rare earth elements penetrate deep into the molten steel ladle, promoting a more uniform distribution and reducing the chance of reaction with air (oxidation). The reaction is highly exothermic, which helps keep the metal molten during addition.
Step 4: The Metallurgical Role of Rare Earths – “Purification and Modification”
Once dissolved in the molten steel, the rare earth elements perform two vital functions:
– Purification (Tramp Element Control):
Rare earths have a very high affinity for elements like Sulfur and Oxygen .
They react with sulfur to form Rare Earth Oxysulfides (e.g., Ce₂O₂S, La₂O₂S).
They also react with oxygen to form Rare Earth Oxides (e.g., Ce₂O₃).
Why is this good? The natural impurity in steel is Manganese Sulfide (MnS). MnS inclusions are soft and elongated during the rolling process, creating weak points and initiating fatigue cracks. Rare Earth inclusions are hard, small, and globular.
– Inclusion Modification:
The newly formed rare earth inclusions are thermally stable (do not melt or change shape during rolling).
Because they are small and globular, they do not create stress concentrators like the elongated MnS inclusions. Instead, they are uniformly dispersed in the steel matrix.
This “modification” of the inclusion morphology dramatically improves the steel’s homogeneity, toughness, and most importantly, its fatigue resistance.
Step 5: Continuous Casting
The rare earth-treated molten steel is cast into long, rectangular sections called blooms using a continuous caster. The process is carefully controlled to prevent segregation (uneven distribution of elements) and ensure a sound, defect-free internal structure.
Step 6: Reheating and Hot Rolling
The blooms are reheated to about 1200°C and then passed through a series of rolling mills, including a universal mill, to transform them into the final rail profile (head, web, and foot). The homogeneous structure achieved by rare earth addition allows for a more consistent deformation during rolling.
Step 7: Heat Treatment – Head Hardening
For high-speed rail applications, the rails undergo a premium heat treatment process called Quench and Self-Temper (QST) or head hardening.
The rail head is rapidly cooled with a spray of compressed air or misted water.
This transforms the microstructure at the head into a very fine, strong, and wear-resistant pearlite, while the web and foot remain tougher.
The clean, rare earth-modified steel responds better to this heat treatment, resulting in a more uniform and controlled hardened layer with superior properties.
Step 8: Straightening, Machining, and Inspection
The rails are straightened, the ends are precision-milled for welding, and they undergo 100% automated ultrasonic inspection to detect any internal flaws. The rigorous quality standards for high-speed rail (such as Chinese Standard TB/T 3276) are applied.
In conclusion, Baotou Steel’s integration of rare earth elements is a sophisticated metallurgical technique that acts at the microscopic level to purify the steel and modify harmful inclusions. This process is a key factor in producing the world-class
