Several methods for ultra-fine grain of steel

1. Metallurgical processing to refine grains

The traditional grain refining method in the casting process is mainly realized by adding a nucleating agent for metamorphism treatment, and by providing a large number of dispersed particles to promote non-uniform nucleation, the molten steel is solidified to obtain more fine crystal grains. In addition, alloying can also effectively refine the grain of steel: on the one hand, certain elements, such as Mn, Cr, etc., can lower the phase transition temperature, refine the grain and refine the phase change or phase change. Microalloyed carbonitrides; on the other hand, certain strong carbonitrides form micro-nano-scale compounds with carbon or nitrogen in steel, which strongly impede the growth of grains and promote the formation of a large number of Non-uniform crystal nucleus to refine grains.
At present, in the practice of controlled rolling and controlled cooling, a large number of microalloying elements such as Nb, V, Ti, B are used to form carbon, nitrogen and carbon and nitrogen compounds in steel, which are used to dissolve under different conditions. And precipitation mechanism to achieve the purpose of fine grain strengthening and precipitation strengthening. Nb often forms Nb(C,N) compounds in steel, which prevents the austenite recovery and recrystallization from being the strongest. When controlling rolling, it produces significant grain refinement and certain precipitation strengthening effect; Ti is at high temperature. Dispersion by TiN is also very effective in preventing grain growth of austenite; V also has a strong affinity for carbon, which can effectively prevent austenite grain growth, and V has obvious at hot rolling temperature. Precipitation strengthening effect; nitrogen plays a strong role in solid solution and precipitation of carbonitride in microalloyed steel. Increasing the nitrogen content in steel can effectively play the role of microalloying elements, and can be strengthened with precipitation of N, Nb Grain refinement is used in combination.
Metallurgical treatment of grain refinement is good, and it is one of the most widely used steel grain refining methods, but the method can only be applied to a limited number of specific steel types.

2. Deformation heat treatment to refine grains

The deformation heat treatment is a means for organically combining solid phase transformation or recrystallization with mechanical deformation to heat the material, and is extremely effective for material microstructure refinement. By using the deformation heat treatment, the dual purpose of forming and improving the microstructure can be achieved at the same time, so that the workpiece can obtain excellent strength and toughness.
There are various methods for heat treatment of deformation, which are generally classified into high temperature deformation heat treatment and low temperature deformation heat treatment. The high temperature deformation heat treatment is to heat the steel to a temperature slightly higher than AC3 for a period of time to achieve complete austenitization, and then to make the austenite initial recrystallization with a large reduction, and the grain has not yet Quenching before starting to grow, resulting in a finer quenched structure. The low-temperature deformation heat treatment is to heat the steel after quenching to a point below the transformation point for deformation under a large reduction, and then heat it to a temperature lower than AC3 for a short period of time, and then rapidly quench the austenite to achieve grain refinement.
The deformation heat treatment fine grain technology is the most used grain refining method in industrial production, and is easy to be popularized in industrial production.

3. Magnetic field or electric field refines grains

A strong magnetic field or electric field is an important factor affecting the phase transition of a metal: since different phases have different magnetic permeability or dielectric constant, the electromagnetic field will affect its Gibbs free energy and thus the ya phase transition temperature. In the hot rolling process, the application of a magnetic field or an electric field intermittently can change the AC3 temperature, repeat the austenite-ferrite transformation, and promote the ferrite grain refinement. The external magnetic field or electric field will increase the phase change driving force from austenite to martensite during quenching and cooling, and the same effect as increasing the degree of subcooling can be obtained, thereby increasing the nucleation rate of martensite and reducing its growth. Speed, to achieve the purpose of organizational refinement.

4. Ball milling to refine grains

Ball milling refers to placing large materials into a high-energy ball mill. The material is refined by grinding and impacting between the medium and the material. The product is generally powder, the shape is irregular, and the surface may be chemically reacted with the medium. Contamination, due to multiple deformations, hardening and fracture, there are a large number of defects, so the surface defects are many and the activity is extremely high.

5. Amorphous crystallization and grain refinement

Amorphous crystallization is usually composed of two processes: obtaining and solidifying amorphous solids: amorphous solids can be prepared by techniques such as melt chilling and high-speed direct current sputtering. Crystallization is usually carried out by isothermal annealing. Gradual annealing, pulse annealing and other methods have also been developed.

6. Strong plastic deformation to refine grains

The strong plastic deformation and grain refining method currently has an equal channel extrusion method, a high pressure torsion method, a cumulative stack rolling method, and a multidirectional compression method. However, each method has certain limitations and the size that can be processed is limited.
Grain refinement, as one of the most basic methods to improve the performance of steel materials, can improve the strength of steel without reducing its toughness. It is one of the important research directions of material workers in the steel field.

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