The role of rare earth in steel
Rare earth elements are a general term for 17 special elements. It is named because the Swedish scientists applied rare earth compounds when extracting rare earth elements, so they are named rare earth elements. Due to the variety of rare earth elements, they also have different roles in steel. It can be roughly divided into the following types: purification, metamorphism, grain refinement and microalloying.
(1)Purification
The purification of rare earth in steel is mainly manifested in the deep reduction of oxygen and sulfur content. Reduce the harmful effects of low melting point elements such as phosphorus, sulfur, hydrogen, arsenic, antimony, bismuth, lead and tin. The chemical properties of rare earth metals are unusually active. At the temperature of the molten steel (1550-1600), it acts with harmful impurities such as oxygen and sulfur. A compound having a small density and a high melting point is formed and removed from the molten steel, resulting in a decrease in the content of impurities in the molten steel. A certain amount of rare earth can interact with impurities such as arsenic, antimony, bismuth, lead, and tin in steel to form a compound with a higher melting point. Rare earth reduces the diffusion coefficient of hydrogen and delaying the enrichment of hydrogen in the plastic zone at the crack tip. Thereby, the incubation period and the rupture time of the crack propagation are prolonged.
(2)Metamorphism
The addition of rare earth to steel can change the nature, morphology and distribution of inclusions. Inclusions in steel are harmful substances that affect the properties of steel. Therefore, it should be removed as much as possible during steel making. Calcium treatment has been considered as a more economically viable measure to obtain low-impurity steel. However, this process requires higher cost ladle refractories, expensive ladle flue gas collection systems, new cover slag systems, and the like. So the rare earth treatment is ideal.
Morphological control of inclusions is one of the main roles of rare earth in steel. Rare earth can control the form of sulfur and oxygen inclusions. Final deoxygenation with a small amount of AI and addition of rare earth. Will form a spherical inclusion with a high melting point and randomly distributed in the crystal. Substituting the second type of sulfide distributed along the grain boundaries. When the amount of rare earth added is appropriate, the rare earth sulfide can completely replace MnS. When the rare earth compound is deformed by hot working, it still maintains a fine spherical shape and a spinning cone, and is uniformly distributed in the steel. Elimination of the existing inclusions, which are distributed along the direction of rolling of the steel, in the form of long strips of MnS. Significantly improve lateral toughness, high-temperature plasticity, weldability, fatigue properties, atmospheric corrosion resistance, etc. The coefficient of thermal expansion of rare earth inclusions is similar to that of steel. It can avoid the large additional stress around the inclusions when the steel is hot-processed and cooled, which is beneficial to improve the fatigue strength of the steel. The deterioration of inclusions increases the ability of inclusions and grain boundaries to resist crack formation and expansion.
(3)Grain refinement
The tiny solid particles of the rare earth compound provide a heterogeneous nucleus or segregate at the crystallization interface. It hinders the growth of the unit cell and provides better thermodynamic conditions for steel graining. Therefore, the addition of rare earth to steel can refine the solidification structure of steel, thereby improving the performance of steel.
Studies have shown that the use of rare earth to treat different sulfur-containing cast steels will have a significant influence on their solidification characteristics, macroscopic and microstructure. The effect of rare earth on the crystal structure of high-sulfur cast steel is mainly to make the grain thinner and the equiaxed crystal ratio increase. The mechanism is that the rare earth compound acts as a non-spontaneous core of crystallization. The effect of rare earth on the crystal structure of low-sulfur cast steel is mainly to refine the secondary dendrite spacing.
(1)Purification
The purification of rare earth in steel is mainly manifested in the deep reduction of oxygen and sulfur content. Reduce the harmful effects of low melting point elements such as phosphorus, sulfur, hydrogen, arsenic, antimony, bismuth, lead and tin. The chemical properties of rare earth metals are unusually active. At the temperature of the molten steel (1550-1600), it acts with harmful impurities such as oxygen and sulfur. A compound having a small density and a high melting point is formed and removed from the molten steel, resulting in a decrease in the content of impurities in the molten steel. A certain amount of rare earth can interact with impurities such as arsenic, antimony, bismuth, lead, and tin in steel to form a compound with a higher melting point. Rare earth reduces the diffusion coefficient of hydrogen and delaying the enrichment of hydrogen in the plastic zone at the crack tip. Thereby, the incubation period and the rupture time of the crack propagation are prolonged.
The addition of rare earth to steel can change the nature, morphology and distribution of inclusions. Inclusions in steel are harmful substances that affect the properties of steel. Therefore, it should be removed as much as possible during steel making. Calcium treatment has been considered as a more economically viable measure to obtain low-impurity steel. However, this process requires higher cost ladle refractories, expensive ladle flue gas collection systems, new cover slag systems, and the like. So the rare earth treatment is ideal.
Morphological control of inclusions is one of the main roles of rare earth in steel. Rare earth can control the form of sulfur and oxygen inclusions. Final deoxygenation with a small amount of AI and addition of rare earth. Will form a spherical inclusion with a high melting point and randomly distributed in the crystal. Substituting the second type of sulfide distributed along the grain boundaries. When the amount of rare earth added is appropriate, the rare earth sulfide can completely replace MnS. When the rare earth compound is deformed by hot working, it still maintains a fine spherical shape and a spinning cone, and is uniformly distributed in the steel. Elimination of the existing inclusions, which are distributed along the direction of rolling of the steel, in the form of long strips of MnS. Significantly improve lateral toughness, high-temperature plasticity, weldability, fatigue properties, atmospheric corrosion resistance, etc. The coefficient of thermal expansion of rare earth inclusions is similar to that of steel. It can avoid the large additional stress around the inclusions when the steel is hot-processed and cooled, which is beneficial to improve the fatigue strength of the steel. The deterioration of inclusions increases the ability of inclusions and grain boundaries to resist crack formation and expansion.
(3)Grain refinement
The tiny solid particles of the rare earth compound provide a heterogeneous nucleus or segregate at the crystallization interface. It hinders the growth of the unit cell and provides better thermodynamic conditions for steel graining. Therefore, the addition of rare earth to steel can refine the solidification structure of steel, thereby improving the performance of steel.
Studies have shown that the use of rare earth to treat different sulfur-containing cast steels will have a significant influence on their solidification characteristics, macroscopic and microstructure. The effect of rare earth on the crystal structure of high-sulfur cast steel is mainly to make the grain thinner and the equiaxed crystal ratio increase. The mechanism is that the rare earth compound acts as a non-spontaneous core of crystallization. The effect of rare earth on the crystal structure of low-sulfur cast steel is mainly to refine the secondary dendrite spacing.
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