1. The term "rare earth" refers to the group Ⅲ B lanthanide elements in the periodic table and scandium and yttrium which are similar in chemical properties to lanthanide elements, totaling 17 elements. It was discovered in 1794 by johan gado1in, a Finnish scholar. At that time, yttrium earth with similar mineral composition as "Earth" was found in the ores of Sweden, and it was thought to be rare, so it was named "rare earth". Since then, many elements of the same kind have been found, collectively known as rare earth. But later research found that the abundance of rare earth in the earth's crust is much more than people thought. For example, cerium is much more than tin, yttrium is also much more than lead, and even rare earth elements with less abundance are more than platinum group elements, indicating that rare earth is not rare. It's not earth, it's all metal.

China is rich in rare earth resources, which no other country in the world can match. The proven industrial reserves are 36 million tons, accounting for about 80% of the world's total, with a wide variety and concentrated distribution. The reserves of Bayan Obo mine in Baotou City account for more than 95% of China's reserves. So there is the saying that "the world's rare earth is in China, and China's rare earth is in Baotou".

According to their differences in properties and requirements of separation process, rare earth elements are generally divided into two groups: light rare earth and heavy rare earth, of which lanthanum, cerium, praseodymium, neodymium, Ju, samarium and europium are light rare earth. Rare earth elements are typical metal elements. Their metal activity is second only to alkali metals and alkaline earth metals. They are more active than other metal elements and can be combined with many elements. Moreover, the ignition point of rare earth metals is very low, such as cerium 165 ℃, neodymium 270 ℃, which is very easy to react with oxygen. All the rare earth metals can be oxidized into re203 type oxides in the air of 180 ℃ - 200 ℃. The melting point of the rare earth oxides is very high, and the negative value of the free energy is very large, which shows that they are very stable compounds. Due to the special properties of rare earth elements, the use of rare earth is determined. The main applications in iron and steel industry are rare earth ferrosilicon alloy (containing light rare earth mixed metal 20% - 45%), rare earth ferrosilicon alloy (rare earth metal 6% - 25%, magnesium 7% - 12%), heavy rare earth ferrosilicon alloy (containing yttrium mixed rare earth more than 60%). Mixed rare earth metal (containing more than 95% light rare earth), rare earth ferrosilicon alloy rich in cerium or lanthanum (CE accounting for 70% or La accounting for more than 50%). There are two kinds of commonly used in steel-making production, one is rare earth alloy, block rare earth ferrosilicon alloy, which was used for large ladle input, large ladle pressing, powder shape is generally used for powder injection in large ladle, powder injection in mold casting, and other methods to join steel; the other is to mix rare earth metals, and make wires (φ mm - φ mm) or rods (≥ φ mm), and wires are used for ladle, intermediate injection pipe or continuous casting crystallizer, which are fed by wire feeder The rod is melted into the steel by hanging in the die. As a new type of linear additive material, rare earth metal cored wire will be further developed because of its wide application in steel-making.

3. Mechanism of rare earth action in steel

3.1 microalloying action

microalloying action of rare earth elements

it is preliminarily recognized that the main reason is that the segregation of rare earth atoms on the grain boundary interacts with other elements, which causes the changes of the structure, chemical composition and energy of the grain boundary, affects the diffusion of other elements and the nucleation and growth of new phases, and then leads to the changes of the structure and properties of the steel. The content of rare earth metals in steel varies with different steel grades, smelting methods and adding methods. The direct relationship between the content of rare earth in steel and microalloying remains to be studied.

3.2 interaction with other harmful elements

a certain amount of rare earth can interact with phosphorus, arsenic, tin, antimony, bismuth, lead and other low melting point harmful elements in steel. On the one hand, rare earth can form compounds with high melting point with these impurities; on the other hand, it can inhibit the bias of these inclusions on the grain boundary. For example, the existence of hot brittleness in steel is due to the fact that there are some low melting point metal elements in the steel. When rare earth is added to the molten steel, a high melting point metal compound will be formed, which will not melt in the steel but enter the slag, which will play a role of purification and reduce the impurities in the steel, thus overcoming the hot brittleness.

3.3 the thermodynamic analysis of desulfuration and deoxidation of rare earth elements and a large number of studies on rare earth inclusions in steel show that the content of [O] and [S] in steel is in a certain range, and when rare earth is added to the molten steel, it is very easy to generate oxygen sulfide of rare earth. When the oxygen content in the steel drops below 201ppm, the re added into the molten steel forms the re203s type inclusions first, and then the re3s4 or res type sulfides. These sulfides may be wrapped around the oxygen sulfides to form composite inclusions or rare earth silicate compounds. They have high melting point and are very stable and spherical. After the molten steel is properly killed, these rare earth oxides Sulfide or rare earth silicate compounds will be removed from the steel, thus purifying the molten steel. The effect of rare earth in steel is 90% through the control of sulfide form. When re / S is 2.7-3.0, the control effect of sulfide morphology is better.

3.4 hydrogen trapping action rare earth can absorb a large amount of hydrogen and can be made into hydrogen storage materials. When rare earth is added to steel, the brittleness and white spot caused by hydrogen in steel can be restrained. It has been shown that rare earth can reduce the diffusion coefficient of hydrogen, delay the enrichment of hydrogen in the plastic zone with good crack, thus prolonging the incubation period and fracture time of crack growth. Therefore, rare earth can inhibit the hydrogen embrittlement of steel.

3.5 Dispersion Hardening

spraying rare earth oxide (CeO2) powder into the molten steel can improve the strength and toughness of the steel, reduce the brittle transition temperature and improve the permanent strength of the steel. The reason is that on the one hand, ce02 can be used as crystal nucleus to refine as cast grains; on the other hand, the dispersion of ce02 particles can improve the resistance of grain boundary to dislocation.

3.6 modified inclusions

rare earth is added into the molten steel to form spherical rare earth sulphide or sulfur oxide, replacing the long strip MNS inclusions which are easy to form, so that the shape of sulphide can be controlled, the thermoplasticity of steel, especially the transverse impact toughness, and the anisotropy of steel can be improved. Rare earth makes alumina inclusions with high hardness turn into spherical sulfur oxide and rare earth aluminate, which is beneficial to improve the fatigue property of steel.

The influence of rare earth on the properties of steel

the varieties of rare earth steel developed at present are: rare earth niobium heavy rail, high toughness pressure vessel steel, 45kg bridge plate, rare earth ship plate steel, rare earth axle steel, large bridge steel, etc. These steels have been improved in properties and applications due to the presence of rare earth elements. It shows that rare earth has a great influence on the properties of steel.

4.1 effect of rare earth on rail steel

when the content of rare earth (CE, La, PR, mixed rare earth) in rail steel is more than 0.029%, the following effects can be produced:

① rare earth can delay the initiation and propagation of contact fatigue crack of rail steel and the occurrence of rail surface peeling. ② Rare earth can obviously reduce the contact fatigue penetration angle and penetration depth of rail steel. ③ Rare earth can reduce the plastic deformation range of rail steel contact fatigue surface and improve the work hardening effect. ④ Rare earth has the functions of purifying molten steel, modifying inclusions and microalloying. ⑤ It can not only reduce the stress concentration area, but also refine the structure, improve the strength and enhance the deformation resistance of rail steel. ⑥ Because rare earth is very easy to oxidize, and with oxidation synthesis oxide film, it adheres to the rail surface, the resulting "white lubrication" effect can not only reduce the friction coefficient, but also improve the surface bonding strength, thus improving the fatigue and wear of the rail, and the wear resistance is twice that of the common rail.

4.2 effect of rare earth on 60CrMnMo

when the content of rare earth in steel reaches 0.05% - 0.07% after rare earth treatment, the thermal fatigue life and plasticity of hot rolled roll steel can be significantly improved.

4.3 the effect of rare earth on 16mnre

significantly improved the toughness and plasticity of steel, especially the transverse toughness, plasticity and stamping performance of steel. This steel is widely used in automobile, bridge, shipbuilding, container and construction industry.

4.4 the effect of rare earth on 09mnre

significantly improve the plasticity of automobile and vehicle steel. It is used for medium-sized automobile engine baffle and carriage frame edge plate, etc.

4.5 The effect of rare earth elements on petroleum drilling pipe steel the main role of rare earth elements in steel is to remove the impurity elements in the molten steel, especially after rare earth elements are added to the steel, the sulfur content can be reduced from 0.025% to 0.01%, and the residual inclusions, mainly the shape of manganese sulfide, can be changed, so as to improve the strength of the steel. This is because the residual rare earth inclusions are hard and spherical during rolling, which is not easy to form An elongated sheet that causes cracks. Secondly, rare earth can play a role of dehydrogenation in steel when there are harmful gases such as hydrogen. These characteristics are very important for oil well drilling steel. Adding mixed rare earth metals can improve the service life of drilling pipe steel. There are still many important aspects not discussed, such as rare earth nano medical materials, sensor materials, magnetic storage materials, etc. to be described later; but only from these examples introduced in this paper, we can see that rare earth nano particles and technology play an innovative role in many fields, such as materials, energy, information, environmental protection, etc., with which we can open up countless new materials that have never been before; works in application Use and effect are immeasurable.

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