With the development of modern engineering technology, the requirements for the comprehensive performance of steel are becoming increasingly strict, and accordingly, the requirements for the material of steel are getting higher and higher. Non-metallic inclusions exist in the steel as an independent phase, destroying the continuity of the steel matrix, increasing the inhomogeneity of the structure in the steel, and seriously affecting various properties of the steel.
For example, non-metallic inclusions lead to stress concentration and fatigue fracture; inclusions with a large number and uneven distribution will significantly reduce the plasticity, toughness, weldability, and corrosion resistance of steel; Hot brittleness. Therefore, the quantity and distribution of inclusions are identified as important indicators for assessing the quality of steel and are listed as routine inspection items for high-quality steel and high-grade high-quality steel.
The nature, shape, distribution, size, and content of non-metallic inclusions are different, and the influence on the properties of steel is also different. Therefore, improving the quality of metal materials, producing clean steel, or controlling the properties and required forms of non-metallic inclusions is a difficult task in the process of smelting and ingot casting. For metallographic analysts, how to correctly judge and identify non-metallic inclusions has therefore become very important.
1. Source classification of non-metallic inclusions in steel
1) Endogenous inclusions
During the steel smelting process, the deoxidation reaction will produce oxides and silicates, and other products. If they do not float out before the molten steel solidifies, they will remain in the steel. Oxygen, sulfur, nitrogen, and other impurity elements dissolved in molten steel are precipitated from the liquid phase or solid solution in the form of compounds due to the decrease in solubility and finally remain in the steel ingot. It is the metal in the Inclusions formed during melting. The distribution of endogenous inclusions is relatively uniform, and the particles are also small. Correct operation and reasonable process measures can reduce their number and change their composition, size, and distribution, but generally speaking it is unavoidable.
2) Foreign inclusions
The slag suspended on the surface of molten steel during steel smelting and pouring, or the refractory materials or other inclusions dropped from the inner walls of steelmaking furnaces, tapping troughs, and ladles, etc., remain in the steel if they are not removed in time before the molten steel solidifies. It is an inclusion produced by the contact between metal and foreign substances during the smelting process. For example, the sand and lining on the surface of the charge react with the molten metal to form slag and stay in the metal, including the added flux. Such inclusions are generally irregular in shape and relatively large in size and are also called coarse inclusions. Such inclusions can be avoided with proper handling.
2. non-metallic inclusions in steel are classified according to their chemical composition
Non-metallic inclusions in steel are mainly divided into three categories according to their chemical composition.
1) Oxide-based inclusions Simple oxides include FeO, Fe2O3, MnO, SiO2, Al2O3, MgO and Cu2O.
In cast steel, inclusions are more common when deoxidized with ferrosilicon or aluminum. Steel often aggregates in spherical form and distributes in clusters in granular form. Complex oxides, including spinel inclusions and various calcium aluminates and calcium aluminates. Silicate inclusions also belong to complex oxide inclusions, such inclusions include 2FeO·SiO2 (iron silicate), 2MnO·SiO2 (manganese silicate), and CaO·SiO2 (calcium silicate). During the solidification process of such inclusions, due to the fast cooling rate, some liquid silicates have no time to crystallize, and all or part of them are preserved in the steel in the form of glass.
Figure 1 Aluminum oxide and calcium aluminate inclusions under scanning electron microscope
Fig.2 Silicate and manganese sulfide inclusions under scanning electron microscope
2) Sulfide series inclusions
Mainly FeS, MnS, CaS, etc.
Since FeS with a low melting point is easy to form hot embrittlement, it is generally required to contain a certain amount of manganese in steel, so that sulfur and manganese can form MnS with a higher melting point to eliminate the harm of FeS. Therefore, the sulfide inclusions in steel are mainly MnS.
The forms of sulfide inclusions in as-cast steel are usually divided into three categories:
①The shape is spherical, and this kind of inclusion usually appears in steel with incomplete deoxidation of ferrosilicon;
②Observe under the optical microscope the extremely fine needle-like inclusions in the form of chains;
③It is blocky and irregular in shape, and it appears when excessive aluminum is deoxidized.
3) Nitride inclusions
Nitrides such as AlN, TiN, ZrN, and VN are formed when elements with greater affinity to nitrogen are added to the steel. During the tapping and casting process, the molten steel is in contact with air, and the number of nitrides increases significantly.
3. Classification according to the plastic deformation ability of inclusions
1) Brittle inclusions
The shape and size of this type of inclusion do not change during thermal processing, but they may be arranged in series or in the form of point chains along the processing direction. Al2O3 and Cr2O3 belong to this type of inclusion.
2) Plastic inclusions
Such inclusions have good ductility during thermal deformation and extend into strips along the deformation direction. Belonging to this category are sulfides and ferromanganese silicates with low SiO2 content (40%~60%).
3) Spherical invariant inclusions
It is spherical in cast state and remains spherical after thermal processing, such as SiO2 and silicates with higher SiO2 content (>70%).
4) Semi-plastic inclusions
Refers to various complex phases of aluminosilicate inclusions. The matrix aluminosilicate has plasticity, and it will produce plastic deformation during hot working, but the precipitated phase contained in it, such as alumina, is brittle, and it remains in its original shape or just distanced during processing.
4. Identification of inclusions
Early workers mainly used optical microscopes with X-ray structure analysis and chemical composition analysis, and accumulated valuable experience and rich data. In recent years, the use of electron probes to analyze the micro-composition of inclusions is increasing. There are currently two general methods for identifying inclusions.
1) Combination of metallographic method and micro-area component analysis
After the undetermined inclusions are selected in the metallographic observation, the electronic probe (EPMA) is used for micro-area component analysis or the scanning electron microscope (SEM) with its own energy spectrum analysis (EDS) is used for component analysis. Generally, the constituent elements and approximate composition of inclusions with a size greater than 1um can be determined, and more intuitive results can be obtained if surface scanning of individual elements is used. Figure 4 is a surface analysis spectrum of inclusion in Q460 steel using a scanning electron microscope. The four elements of sulfur, manganese, silicon, and iron are scanned sequentially. From the scanning results, it can be inferred that the inclusion in the bright field observation For MnS, SiO2, and FeS, the composition analysis is carried out by energy dispersive spectroscopy (EDS), and the mass fraction of each element can also be obtained directly.
Figure 3 Scanning Electron Microscopy of inclusions
2) Optical metallography
Observe the color, shape, size, and distribution of inclusions in the bright field under an optical microscope; observe the inherent color and transparency of inclusions in a dark field; observe the various optical properties of inclusions in cross-polarized light to judge Inclusion type. According to the distribution and quantity of inclusions, the corresponding grades are evaluated, and their influence on the properties of steel is judged. At present, there are many methods for testing and researching non-metallic inclusions in steel, including chemical methods, petrographic methods, metallographic methods, electronic probes, and electronic scanning methods. Metallographic identification of inclusions is based on the shape, distribution, and optical characteristics of the inclusions in bright field, dark field, and polarized light, and compared with known inclusion characteristics to determine its type. If necessary, the microhardness of the inclusions or the ability to withstand chemical corrosion can be determined.
Although the content of non-metallic inclusions in steel is small, it has a great influence on the performance of steel, so it must be detected qualitatively and quantitatively. According to the different optical characteristics of the inclusions under the microscope, the non-metallic inclusions in the steel can be qualitatively identified, and the level of the inclusions can be quantitatively assessed in combination with relevant standards and relevant micro-area component analysis, and the quality of the steel can be judged comprehensively, and then find out the law, improve the process, reduce the content of harmful inclusions as much as possible, and improve product quality.