Properties and metallographic features of inclusions in steel

The presence of non-metallic inclusions in steels greatly affects the comprehensive mechanical and physical properties of materials. Even if the content of inclusions is very low, it can reduce the fatigue strength of steel and cause a series of performance problems. The main components of non-metallic inclusions in steels are oxides and sulfides generated by oxidation or reaction of impurities in steel during smelting, rolling and heat treatment processes. Non-metallic inclusions can be divided into groups according to the method of component identification, component oxidation, component distribution, component shape and size, component distribution basic form, and component surface state.

Metal nonmetallic inclusions are mainly oxides and sulfides which together form a large group of inclusions. Oxide inclusions mainly include FeO, MnO, MgO, CaO, Al2O3, etc. Sulfides mainly include FeS, FeS2, CaS and MnS. In addition, there are also Al2O3, SiO2, TiO2 and a small amount of P2O5, etc. These non-metallic inclusions are usually three-phase systems with multi-component components, and their components and proportions vary with different steel smelting processes.

The shapes and sizes of non-metallic inclusions in different types of steels are also different. In low carbon steel, most of the non-metallic inclusions are irregular particles, with a large number of aqueous inclusions, which have a great influence on the roughness and fatigue strength of the material. The middle and high carbon steel contains much fewer inclusions, but some inclusions are wedge-shaped or plate-shaped, and most of them are unoxidized inclusions. Plate-like inclusions can have a very significant impact on steel performance.

Different smelting processes can also cause different effects on inclusions. In the LC converter smelting process, it is easy to form a large number of manganese-based inclusions due to MnO decomposition. In addition, the presence of inclusions can also cause different distributions in different parts of the workpiece due to differences in the smelting process. For example, in the steel of the continuous casting section, the center part of the workpiece has more inclusions, while the two sides have fewer inclusions.

In addition, the effect of non-metallic inclusion on physical and mechanical properties of steel can be determined by its composition, size, shape and distribution. Generally speaking, the higher content of inclusions in the steel, the worse the mechanical properties of the steel. In addition, the shape, size and distribution of inclusions also have a significant impact on the mechanical properties of steel.

In conclusion, non-metallic inclusions in steel are ubiquitous and have a great influence on the mechanical properties of steel. Therefore, the study of non-metallic inclusions in steel is of great significance. In the process of study, the gold phase technique is an important tool to determine the types, shapes, sizes and distributions of inclusions, measure the content of inclusions in steel, and observe and analyze the physical and mechanical properties of materials affected by different inclusions.

Inclusions in Steel

Inclusions are impurities, flaws, or foreign materials contained in steel. They also known as slag inclusion, a type of secondary metallurgical generated during steel production. Inclusions exist as a combination of solid and liquid phases, thus they have impact on the quality and performance ofsteel products.

Inclusions are detrimental to the steel and could damage the microstructure or physical properties of the material.

They may cause surface smears, affect the hot working behaviour of steel, affect the weldability and machinability, reduce toughness and even become initiation sites for crack propagation.

In addition, inclusions are important for the assessment of the quality of the steel. Inclusions exist as different types. Graphite, sulphides, and oxides can exist as both solid and liquid phases and may occur as spheres, acicular particles, granules, cracks and intergranular tracks. All these properties are used to identify the type and size ofinclusion.

To study the properties and morphologies of inclusions, metallographic examination is used. Field metallography has many advantages including the flexibility of sample preparation and examination. Furthermore, the macroscopic and the microstructural informationcan be obtained at the same time.

Using field metallography, an overview of the presence, type and distribution of inclusion is obtained. With the aid of a scanning electron microscope (SEM) or transmission electron microscope (TEM), further examination of the inclusion morphology and composition can be made.

In conclusion, inclusions are a critical factor in steel production and quality. Metallographic examination is an effective tool to study their properties, types and distribution. Different types of microscopy techniques can be used for further in-depth studies.