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Application of rare earth in steel making
The application of rare earth elements in steel has a long history, spanning nearly three decades. Through extensive research on the behavior and mechanisms of rare earth metals in steel, scientists have clearly defined their roles. Experimental studies on the addition process have enabled precise control over the type, amount, and conditions for introducing rare earth elements into the steelmaking process. By the late 1980s, there were no major technical barriers to using rare earths in steel production. China's rare earth steel output grew significantly, from 110,000 tons in 1985 to nearly 600,000 tons by 1997, with more than 80 different grades produced. During the "Eighth Five-Year Plan" period, one company alone—WISCO—produced 1.6 million tons of rare earth steel, generating economic benefits of 320 million yuan, social benefits of 1.83 billion yuan, and saving over 50 million U.S. dollars in foreign exchange.
When rare earth elements are added to steel, they serve multiple functions such as deoxidation, desulfurization, and modifying inclusions, which help purify and refine the material. In certain types of steel, rare earths can also act as microalloying agents. They enhance properties like oxidation resistance, high-temperature strength, plasticity, fatigue life, corrosion resistance, and crack resistance.
**Main Roles of Rare Earth Elements in Steel:**
**Purification:** Adding rare earth to molten steel helps replace oxygen and sulfur in inclusions such as manganese sulfides, alumina, and aluminosilicates, forming rare earth compounds. These compounds tend to float to the surface and enter the slag, reducing the number of inclusions in the steel and improving its purity.
**Refinement of Microstructure:** The rare earth compounds formed during the reaction with inclusions have higher melting points and precipitate before solidification. These fine particles act as nucleation sites, reducing the degree of supercooling during crystallization. This leads to a finer grain structure, reduced segregation, and improved mechanical properties.
**Inclusion Morphology Control:** When rare earth is added, it replaces manganese sulfide with rare earth oxides or sulfides that are less prone to deformation at high temperatures. These spherical inclusions do not deform during rolling, minimizing their negative impact on the steel’s mechanical properties. As a result, the toughness and fatigue resistance of the steel are enhanced.
**Corrosion Resistance:** In atmospheric corrosion-resistant steel, rare earth elements make the rust layer denser and improve its adhesion to the base metal, preventing oxygen and moisture from penetrating. This reduces the corrosion rate, with rare earth-containing steel showing up to 2.4 times better corrosion resistance compared to non-rare earth steel.
**Improved Mechanical Properties:** In MnNb low-alloy high-strength steel, rare earth improves cold bending, impact, low-temperature impact, and wear resistance, enhancing both processing performance and service life. In railway rails, rare earth additions significantly boost wear and spalling resistance, extending rail life by 1.5 times after years of use.
By leveraging these benefits, the use of rare earth elements in steel continues to play a vital role in modern metallurgy, offering both economic and environmental advantages.