What Are Inclusions in Steel?

A collective term for various non-metallic particles entrained in steel. Steel contains elements such as oxygen, nitrogen, and sulfur. Their solubility in steel is high at high temperatures and very low at room temperature. It precipitates when the steel cools and solidifies and combines with iron and other metals to form various compounds. Called non-metallic inclusions. In addition, foreign materials such as slag, refractories, and sand may also be mixed into the steel to form non-metallic inclusions. ^[1]

Earlier literature referred to non-metallic inclusions in steel as "slag inclusions". This name is easily misunderstood, thinking that non-metallic inclusions are slag mixed into steel. Nowadays, various materials mixed into steel are generally called foreign inclusions, and their shapes are irregular. Inclusions generated by internal physical and chemical reactions are called endogenous inclusions. Their typical characteristics are small size and number. Many, evenly distributed. The process of generating inclusions in steel is roughly as follows: after the deoxidant is added to the molten steel, the deoxidizing element and oxygen chemically react to form an insoluble oxide in the steel; some deoxidizing elements can also combine with sulfur and nitridation to form sulfides and nitrides. . Such compounds are called primary inclusions. With the exception of very few fine particles, most of the primary inclusions can float out of the molten steel and enter the slag. When the molten steel is cooled and solidified, due to the decreased solubility and segregation of oxygen and sulfur, oxides and sulfides are generated during the solidification process, which are called secondary inclusions. Secondary inclusions are difficult to exclude from the steel and remain in the dendrites or eventually precipitate out of the grain boundaries. After the molten steel is deoxidized, it continues to come into contact with air or other oxides such as refractory materials, so that the molten steel reabsorbs oxygen, that is, secondary oxidation occurs. Secondary oxidation is an important source of non-metallic inclusions in finished steel.

The presence of non-metallic inclusions in steel destroys the continuity of the metal matrix and deteriorates the quality of the steel. In special cases, some inclusions are conducive to certain properties of the steel (such as machinability), but this is only under special conditions. Generally speaking, non-metallic inclusions have considerable harm to the mechanical properties, physical properties and chemical properties of steel. In popular terms, steel with more inclusions is "dirty", and pure steel contains very few inclusions. However, pure steel is a relative concept. The cleanliness of steel is related to its use, and it is also related to the shape, particle size, and plasticity of inclusions. Although the quantity is small, the inclusions with larger particles are more harmful than the inclusions with larger numbers but smaller sizes; irregular shapes are more harmful than spherical inclusions. For precise small parts, tiny inclusions on the surface are also very harmful. Therefore, we must study not only how to reduce its content, but also its morphology and distribution. Non-metallic inclusion types can be classified from different viewing angles. According to the chemical composition, it can be divided into oxide inclusions, sulfide inclusions, and nitride inclusions. According to the plasticity of the inclusions, it can be divided into general inclusions, brittle inclusions and non-deformation inclusions. According to the size of the inclusions, it can be divided into large inclusions and micro inclusions. . The nature and morphology of inclusions are related to its composition.

Because non-metallic inclusions have multiple effects on the properties of steel, and the sources of inclusions are various, it is difficult to judge accurately. The study of non-metallic inclusions in steel has always been an important subject in steelmaking. The analysis and identification technology for non-metallic inclusions in steel has been continuously developed with the advancement of microanalytical instruments.

Oxides are the most common inclusions and there are many types. Oxide inclusions are generally classified into the following 4 categories: (1) simple oxides, such as FeO, Fe ₃ O ₄ , Fe ₂ O ₃ , MnO, SiO ₂ , Al ₂ O ₃ , Cr ₂ O ₃ and (Mn, Fe ) O solid solution; (2) silicate; (3) spinel inclusions; (4) caloaluminate. Representative of these important inclusions are alumina, silicate, spinel, and caloaluminate.

There are fewer types of sulfide inclusions, the main one being MnS. MnS cannot be formed in the molten steel. Due to the segregation of sulfur during the solidification of the steel, sulfide inclusions precipitate out of the dendrites. The faster the cooling rate, the smaller the precipitated sulfide particles, but the larger the number. With the difference of oxygen content in steel, there are 3 types of sulfide inclusions in as-cast steel: Type I sulfides are irregularly distributed spheres, the particle size is large, in boiling steel and semi-stable steel with high oxygen content It can be seen that it is precipitated at the same time as the iron crystals at the initial stage of solidification. Type II sulfides are network-like or chain-like, distributed along grain boundaries, and are produced in the final stage of solidification. Type III sulfides are lumps with clearly visible edges, corners, and faces. They are irregularly distributed and appear in steels with excessive aluminum deoxidation. They are the result of sulfide crystals developing during solidification. Most sulfide inclusions have good plasticity and extend into a strip shape along the rolling direction during rolling. Type sulfides can form continuous bands after rolling, so whether in the as-cast or rolled steel, type sulfides are the most harmful to the performance of the steel. ^[2]

Adding nitrogen, Al, V, Ti, Zr, Nb, etc. into the steel can generate their nitrides. The nitride inclusion particles are small, square or polygonal in shape, brittle, and will not deform during pressure processing. Some nitride inclusions also contain carbon and sulfur, which are not pure nitrides.

The presence of non-metallic inclusions in steel is inevitable. However, the source, type, and distribution of inclusions are extremely accidental. The development of new steel grades may also cause the generation of new non-metallic inclusions. Therefore, identification and analysis of non-metallic inclusions is a necessary means to identify inclusions, understand their composition, properties, and origin. The identification and analysis of inclusions can provide a scientific basis for reducing harmful inclusions in the smelting and casting process. The methods of identifying inclusions can be divided into two categories: macroscopic identification and microscopic identification.

Macroscopic identification of non-metallic inclusions in steel

Inspect the fracture or surface of the metal material with the naked eye or low magnification, or inspect the inclusions inside the material by non-destructive inspection methods such as X-rays, -rays, or ultrasound. When the environment of the steelmaking workshop is dirty or the quality of the refractory used is poor, large foreign inclusions visible to the naked eye can be found on the surface of the ingot or slab, and this problem rarely occurs when the production order is good. X-ray fluoroscopy can detect large Al ₂ O ₃ clusters inside the steel. Ultrasonic flaw detection can determine the quantitative distribution of inclusions in steel. Tests show that there is a good correspondence between the results of flaw detection and the number of inclusions measured by quantitative microscopy. The main disadvantage of ultrasonic flaw detection is that it is not sensitive to the detection of sulfide inclusions. Sulfide inclusions can be inspected by sulfur imprinting. The solubility of sulfur in solid steel is very small, and most of them become sulfide precipitation. The sulfur segregation zone shown by sulfur imprints is the location of sulfide inclusions. And because calcium aluminate inclusions are often associated with calcium sulfide, sulfur prints can also reflect the distribution of high calcium inclusions, but the sulfides of Zr, Nb, Cr, and V in alloy steel cannot be reflected on the sulfur prints. The advantage of macro identification is that the distribution of inclusions in the steel can be checked directly, but the disadvantage is that the nature and composition of the inclusions cannot be accurately judged.

Microscopic identification of non-metallic inclusions in steel

Microscopic identification of inclusions has formed a complete comprehensive technology, including metallographic method, petrographic method, X-ray diffraction analysis and electronic probe, scanning electron microscope and transmission electron microscope identification equipment. In the 1950s, the bright field, dark field, and polarized light of optical microscopes were often used for the qualitative identification of inclusions, sometimes combined with chemical corrosion of the sample. The mineral composition of the inclusions was applied by X-ray powder diffraction. After the 1960s, electron microscopy and electron microanalysis techniques were applied to the identification of inclusions, which led to a leap forward in the identification and analysis of inclusions. The transmission electron microscope has a very high resolution (0.2 to 0.3 nm), which is equivalent to a thousand times that of an optical microscope. However, it cannot directly observe the material itself. Thing. Scanning electron microscope resolution (10nm) is not as good as transmission electron microscope, but you can directly observe the sample to get a strong stereoscopic image, and you can get the relative amount of each element in the sample by using the X-ray energy spectrum excited on the sample . Electron probe is the abbreviation of electron probe microanalyzer. It can analyze the element composition in the m volume and directly give an image of the element distribution, so as to judge the mineral composition of the inclusions, which is very helpful for analyzing the composition of complex inclusions.

Quantitative analysis of non-metallic inclusions in steel

For steel production and users, how to quantify the cleanliness of steel is very meaningful. The easiest way is to rate the inclusions and divide them into 4 types according to the characteristics of the inclusions after pressing: Type A is a strip-shaped sulfide, Type B is a chain-like brittle inclusion such as Al ₂ O ₃ , and Type C is plastic deformation Silicate, D type is spherical non-deformable inclusions. The authoritative unit puts forward the inclusion rating chart showing cleanliness as a standard, and the steel to be evaluated is compared with it. With the change of steel varieties and the expansion of uses, the use of this rating method has been unable to correctly express the quality of steel. Another method is to use a metallographic microscope to select a number of fields to measure the number of inclusions, but this is very eye-consuming and inaccurate. Due to the development of quantitative metallographic technology, PASEM (scanning electron microscope for particle analysis) has become the most effective tool for quantitative research of inclusions. It can measure 8 kinds of parameters including the size distribution, area, perimeter, and projection length of the inclusions, and can record the coordinates of the center position of the inclusions, so that the parameters of the inclusions correspond to the scanned images and energy spectrum analysis. deal with. In addition to physical methods for quantifying inclusions, electrolytic separation of inclusions and chemical analysis are still useful methods for analyzing the chemical composition and phase composition of inclusions. The method of phase separation and analysis in high alloy steel still needs to be studied. ^[3]