Microalloying of rare earth in steel
Since the atomic radius of a rare earth metal is very different from the atomic radius of an industrial metal. The solid solubility of rare earth metals in iron liquid is very small, and it is difficult to form a solid solution. Therefore, the alloying effect is extremely poor. However, in the rare earth alloy steel, since the rare earth is mainly segregated at the grain boundary, the structure, chemical composition and properties of the grain boundary are changed. It also affects the diffusion of other elements and the nucleation and growth of new phases, which leads to changes in the microstructure and properties of steel. This change is considered to be the microalloying of rare earths. Let's divide the class.
(1)Solid solubility and solid solution strengthening
It can be seen from the phase diagram of the rare iron phase that the rare earth element is mutually soluble in the iron liquid and the iron atom. However, its partition coefficient in iron-based solid solution is extremely small. During the solidification of molten iron, it is enriched in interdendritic or grain boundaries by the solid/liquid interface. The solid solubility of rare earth elements in iron is difficult to measure by conventional methods. In particular, since rare earth elements have a large chemical affinity with common impurity elements such as sulfur and oxygen in iron. It is easy to react and form inclusions, which affects the accuracy of the determination of rare earth elements in iron. Determination of rare earth alloying amount by internal friction method, X-ray measurement lattice constant method, non-aqueous electrolytic separation of inclusions, and ICP spectrum, etc. It shows that the amount of solid solution of rare earth is basically in the range of parts per million to 100,000 parts, and some still reach a few ten thousandths. Since the radius of the rare earth atom is larger than that of the iron atom, it can provide a strengthening effect on the solid solution.
(2)Improve grain boundaries
Rare earth dissolved in steel are often enriched in grain boundaries by diffusion mechanisms. Reduces the segregation of impurity elements at the grain boundaries and strengthens the grain boundaries. Improves the properties of steel associated with grain boundaries. For example, in 25 MntiB steel, the rare earth is added within a certain quenching temperature range. It will hinder the segregation of boron in the austenite grain boundary. Improve the performance associated with grain boundaries. Such as low temperature brittleness, fatigue properties, grain boundary corrosion, high temperature strength and temper brittleness. Related studies have also found that rare earth have a regional segregation effect that reduces phosphorus, so that phosphorus is no longer concentrated in the grain boundaries. For the 900 quenching, the steel sample of the 650 tempering furnace is cold-embrittled and impact-ruptured at liquid nitrogen temperature. Among them, the steel without La is mainly broken along the crystal. However, a steel containing a suitable amount of La produces a transgranular fracture. Explain that La can prevent embrittlement of steel samples along the crystal fracture.
(3)Decarburization of low carbon and nitrogen
The desolvation of carbon and nitrogen in steel is a possible cause of blue brittleness. Rare earth can significantly reduce the de-dissolved amount of carbon and nitrogen in iron. So that they cannot be desolvated into the internal stress zone or crystal defects. Reduce the number of interstitial atoms in pinning dislocations. This increases the ductility and toughness of the steel. In addition, the rare earth affects the morphology, size, distribution, quantity and crucibility of the carbide, and improves the mechanical properties of the steel.
(4)Affecting phase change and improving organization
The rare earth affects the critical point of steel. Related experiments have observed that the rare earth affects the phase transition temperatures of steels such as Ac, Arn, Ac3, Ar3, M, M, etc. Change the structure of the phase change product. In different rare earth steel, fine cementite, refinement lath martensite substructure or dislocation martensite structure was observed. There is also a phenomenon in which the content and size of the ferrite are changed, and the aggregation and coarsening of the carbide phase are suppressed.
(1)Solid solubility and solid solution strengthening
It can be seen from the phase diagram of the rare iron phase that the rare earth element is mutually soluble in the iron liquid and the iron atom. However, its partition coefficient in iron-based solid solution is extremely small. During the solidification of molten iron, it is enriched in interdendritic or grain boundaries by the solid/liquid interface. The solid solubility of rare earth elements in iron is difficult to measure by conventional methods. In particular, since rare earth elements have a large chemical affinity with common impurity elements such as sulfur and oxygen in iron. It is easy to react and form inclusions, which affects the accuracy of the determination of rare earth elements in iron. Determination of rare earth alloying amount by internal friction method, X-ray measurement lattice constant method, non-aqueous electrolytic separation of inclusions, and ICP spectrum, etc. It shows that the amount of solid solution of rare earth is basically in the range of parts per million to 100,000 parts, and some still reach a few ten thousandths. Since the radius of the rare earth atom is larger than that of the iron atom, it can provide a strengthening effect on the solid solution.
Rare earth dissolved in steel are often enriched in grain boundaries by diffusion mechanisms. Reduces the segregation of impurity elements at the grain boundaries and strengthens the grain boundaries. Improves the properties of steel associated with grain boundaries. For example, in 25 MntiB steel, the rare earth is added within a certain quenching temperature range. It will hinder the segregation of boron in the austenite grain boundary. Improve the performance associated with grain boundaries. Such as low temperature brittleness, fatigue properties, grain boundary corrosion, high temperature strength and temper brittleness. Related studies have also found that rare earth have a regional segregation effect that reduces phosphorus, so that phosphorus is no longer concentrated in the grain boundaries. For the 900 quenching, the steel sample of the 650 tempering furnace is cold-embrittled and impact-ruptured at liquid nitrogen temperature. Among them, the steel without La is mainly broken along the crystal. However, a steel containing a suitable amount of La produces a transgranular fracture. Explain that La can prevent embrittlement of steel samples along the crystal fracture.
(3)Decarburization of low carbon and nitrogen
The desolvation of carbon and nitrogen in steel is a possible cause of blue brittleness. Rare earth can significantly reduce the de-dissolved amount of carbon and nitrogen in iron. So that they cannot be desolvated into the internal stress zone or crystal defects. Reduce the number of interstitial atoms in pinning dislocations. This increases the ductility and toughness of the steel. In addition, the rare earth affects the morphology, size, distribution, quantity and crucibility of the carbide, and improves the mechanical properties of the steel.
(4)Affecting phase change and improving organization
The rare earth affects the critical point of steel. Related experiments have observed that the rare earth affects the phase transition temperatures of steels such as Ac, Arn, Ac3, Ar3, M, M, etc. Change the structure of the phase change product. In different rare earth steel, fine cementite, refinement lath martensite substructure or dislocation martensite structure was observed. There is also a phenomenon in which the content and size of the ferrite are changed, and the aggregation and coarsening of the carbide phase are suppressed.
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