一.What are inclusions
Nonmetallic inclusions are a class of components in steel, which are formed during the solidification and cooling of steel, and undergo a series of changes during cold and hot processing. According to their origin, inclusions are usually classified into two categories: extraneous and endogenous, and can also be classified according to their deformation capacity, morphology and distribution. This section mainly introduces the classification by source. According to the deforming ability of non-metallic inclusion atmosphere, brittle inclusion Al2O3, plastic inclusion FeS,MnS, and non-deforming inclusion,SiO2. Classification by morphology and distribution, Class A, Class B, Class C, Class D.
1.endogenous inclusion
In the process of steel smelting, the deoxidation reaction will produce oxides and other products, if these products do not emerge before the solidification of liquid steel, will remain in the steel;
Mn+FeO → Fe+MnO
Si+2FeO → SiO2+2Fe
2Al+3FeO → 3Fe+Al2O3
Ti+2FeO → 2Fe+TiO2
The impurity elements such as oxygen, sulfur and nitrogen dissolved in the liquid steel are precipitated in the cooling and solidification, or in the solid solution, and finally remain in the ingot. Almost all of them become oxides, sulfides, silicates, etc. These compounds are called endogenous inclusions.
1) Oxides: Common alumina, which is a small insoluble, high-hardness brittle inclusion produced when aluminum is deoxidized. It is broken up during hot processing, and then the small particles are distributed in bands along the processing direction, sometimes several bands in parallel. These banded oxides significantly reduce the fatigue strength of steel.
2) Sulfide: has a high plasticity at high temperatures, can be extended with deformation during hot processing, has a wide range of shape ratio of a single gray inclusion, generally the end is rounded. Common are ferrous sulfide, manganese sulfide, etc., sulfide significantly reduces the transverse properties of steel.
3) Silicate: has high plasticity at high temperatures, can be extended along the processing direction during hot processing, the shape is rough, showing a spindle shape. Most of them are complex inclusions.
4) nitride: it has a high melting point and hardness, does not deform during hot processing, and shows square or regular geometric shapes under the microscope.
5) spot-like non-deformable inclusions: it still maintains a more regular round or oval shape after hot processing.
The distribution of endogenous inclusions is relatively uniform, and the particles are small, and the correct operation and reasonable process measures can reduce its number and change its composition, size and distribution, but it is generally inevitable.
2.Exogenous inclusion
The slag suspended on the surface of molten steel in the process of smelting and pouring, or the refractory materials or other inclusions spalling off the inner wall of steelmaking furnace, steel groove and ladle are not removed in time before the solidification of molten steel, and the mixed refractory materials and slag particles are constantly changed in composition and structure through chemical reaction with molten steel, and become foreign inclusions in steel. It is an inclusion produced by the interaction between metals and external substances during the smelting process.
The general characteristics of such inclusions are irregular shape, relatively large size, and irregular, also known as coarse inclusions. Such inclusions can be avoided with proper operation.
二.The effect of nonmetallic inclusions on steel
1.Service performance
Non-metallic inclusions are distributed in the matrix of the metal, just as many low-strength voids are distributed in the metal, and these voids will flow with the deformation of the metal under the action of external forces, and the voids formed by non-metallic inclusions will be stretched and extended to form cracks, so it will affect the performance of the fracture process (plasticity, toughness, fatigue performance, etc.). The main manifestations are:
Fatigue properties ↓
It is generally believed that inclusion is the origin of steel fatigue failure. Brittle inclusions with weak binding force and large size and spherical non-deformable inclusions have great influence on fatigue properties, and the higher the strength, the greater the harm, as shown in FIG. 5. For high strength steel, if the surface of the member is in good processing condition, the crack initiation from inclusions becomes the main fatigue cracking mode. Small size inclusions may have little effect on crack formation, but are beneficial to fatigue crack propagation.
Non-metallic inclusions are distributed in the matrix of the metal, just as many low-strength voids are distributed in the metal, and these voids will flow with the deformation of the metal under the action of external forces, and the voids formed by non-metallic inclusions will be stretched and extended to form cracks, so it will affect the performance of the fracture process (plasticity, toughness, fatigue performance, etc.). The main manifestations are:
Fatigue properties
Impact toughness Plasticity
The existence of non-metallic inclusions plays an important role in the quality of steel and its products. Sulfide inclusion causes hot brittleness of steel. In the cold state of steel, these inclusions affect its strength and ductility. Some hard angular inclusions obviously reduce the section shrinkage of steel. Non-metallic inclusions have a greater impact on the tensile strength of steel, excellent steel fracture is plastic fracture, when there are inclusions in steel aggregation or inclusions along the grain boundary distribution, the fracture often starts from these places, so that the strength is sharply reduced.
2.processing property
1) In the process of forging and cold working, quenching heating and welding, because the inclusion destroys the continuity of the steel structure, once the tensile stress or shear stress, the steel product will crack along the distribution of inclusions.
2) The surface quality after rolling and the surface roughness of the parts after grinding are reduced.
3.Strength, elongation influence
When the inclusion particles are relatively large (> 10μm), especially when the inclusion content is low. The yield strength and tensile strength of steel are decreased obviously. When the inclusion particles are small to a certain size (< 10μm), the yield strength and tensile strength of the steel will be increased. When the number of small particles dispersed in the steel inclusions increases. The yield strength and tensile strength of steel are increased, but the elongation has a small decrease.
三.Quantitative rating of non-metallic inclusions
During the grading of non-metallic inclusions in steel, Generally, equipment such as optical microscopy, scanning electron microscopy and energy spectrometer will be used to identify the types of non-metallic inclusions in steel according to the optical and morphological characteristics of non-metallic inclusions, and evaluate the level of inclusions with reference to relevant standards (generally according to GB/T 10561-2005 "Determination of non-metallic inclusion Content in steel"), combined with the qualitative and quantitative analysis results of inclusion composition spectrum, accurately determine the composition and source of inclusion, for steelmaking continuous casting to improve the production process, reduce the number of harmful inclusions, improve the purity of steel, and produce high-quality steel to provide a scientific basis.
Quantitative determination is one of the routine inspection items of high quality steel and high quality steel. Under the condition that the type of inclusions is known, the standard grade comparison method is used to judge the quality of steel.
The standard GB/T 10561-2005 "Determination of non-metallic inclusion content in steel" stipulates that the actual area is 0.5mm2, and the actual field of view is 0.71mm2 square field of view.
JK standard rating: Inclusions are divided into four basic types A, B, C and D, which are sulfides, alumina, silicates and spherical oxides.
According to the morphology and distribution of inclusions, the standard spectra are divided into five categories: A, B, C, D and DS. Each type of inclusions is divided into 0.5, 1, 1.5, 2, 2.5, 3, a total of six grades, the greater the number of grades, indicating the greater the number of inclusions, the larger the size. In the GB/T 10561 "Determination of non-metallic inclusion content in steel" standard, there are also relevant provisions for the spacing between two inclusions to avoid the wrong assessment of multiple inclusions of the same type appearing in the same field of view.
Class A (sulfides) : A single gray inclusion with high ductility and a wide range of form ratios, generally having rounded ends.