According to the source and classification of non-metallic inclusions in steel, the methods and quantitative rating standards for identifying non-metallic inclusions in steel are reviewed, and scanning electron micrographs of typical inclusions are given, and the formation mechanism and formation mechanism of different types of inclusions are analyzed. Its basic characteristics under the optical microscope.
With the development of modern engineering technology, the requirements for the comprehensive performance of steel have become increasingly strict, and accordingly the requirements for steel materials have become higher and higher. Non-metallic inclusions exist in the steel as an independent phase, destroy the continuity of the steel matrix, increase the unevenness of the structure in the steel, and seriously affect the various properties of the steel. For example, non-metallic inclusions cause stress concentration and cause fatigue fracture; a large number and uneven distribution of inclusions will significantly reduce the plasticity, toughness, weldability and corrosion resistance of steel; the presence of sulfides in the steel network will cause Hot brittleness. Therefore, the number and distribution of inclusions are recognized as an important index for evaluating the quality of steel, and are listed as the routine inspection items for high-quality steel and high-quality steel.
The nature, shape, distribution, size and content of non-metallic inclusions have different effects on steel properties. Therefore, improving the quality of metal materials, producing clean steel or controlling the properties and required forms of non-metallic inclusions is an arduous 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
1.1 Endogenous inclusions
In the process of steel smelting, the deoxidation reaction will produce products such as oxides and silicates, which will remain in the steel if they do not float before the molten steel solidifies. When the impurity elements such as oxygen, sulfur and nitrogen dissolved in molten steel are cooled and solidified, due to the decrease in solubility, they combine with other elements to precipitate from the liquid phase or solid solution in the form of compounds, and finally stay in the steel ingot. In the smelting process, various physical and chemical reactions formed inclusions. 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 inevitable.
1.2 Foreign inclusions
The slag suspended on the surface of the molten steel during the smelting and pouring process, or the refractory materials or other inclusions dropped from the inner wall of the steelmaking furnace, tapping trough, and ladle, etc. are not removed in time and left in the steel before the molten steel solidifies . It is an inclusion produced by metal contact with external substances during the smelting process. For example, the sand on the surface of the charge and the furnace lining interact with the molten metal to form slag and stay in the metal, including the added flux. The general characteristics of this type of inclusions are irregular shapes, relatively large sizes, and irregularities. They are also called coarse inclusions. Such inclusions can be avoided by correct operation.
2 Non-metallic inclusions in steel are classified by chemical composition
The non-metallic inclusions in steel are classified in detail according to their chemical composition as shown in Figure 1, which are mainly divided into three categories.
2.1 Oxide-based inclusions
Simple oxides include FeO, Fe2O3, MnO, SiO2, Al2O3, MgO and Cu2O. In cast steel, inclusions are more common when ferrosilicon or aluminum is used for deoxidation. In steel, it is often aggregated in spherical shape and distributed in clusters. Complex oxides include spinel inclusions and various calcium aluminates, as well as calcium aluminates (Figure 2b). Silicate inclusions are also complex oxide inclusions, such inclusions include 2FeOSiO2 (iron silicate), 2MnO.SiO2 (manganese silicate) and CaO.SiO2 (calcium silicate), etc. (Figure 3a). During the solidification of steel, due to the fast cooling rate, some liquid silicates are too late to crystallize, and all or part of them are stored in the steel in the form of glass.
2.2 Sulfide system inclusions
Mainly FeS, MnS and CaS etc. Because FeS with a low melting point is easy to form hot embrittlement, it is generally required that a certain amount of manganese be contained in the steel, so that sulfur and manganese form MnS with a higher melting point to eliminate the harm of FeS. Therefore, the sulfide inclusions in steel are mainly MnS (Figure 3b).
The morphology of sulfide inclusions in as-cast steel is generally divided into three categories: ①The morphology is spherical, and this type of inclusion usually appears in steel that is not completely deoxidized with ferrosilicon; ②The chain-like very fine particles are observed under an optical microscope. Needle-shaped inclusions; ③It is massive and irregular in appearance, which appears when excessive aluminum deoxidation.
2.3 Nitride inclusions
When elements with greater affinity for nitrogen are added to steel, nitrides such as AlN, TiN, ZrN and VN are formed. In the process of tapping and casting, the molten steel is in contact with air, and the amount 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 inclusions do not change during hot working, but they may be arranged in series or point chains along the processing direction. Al2O3 and Cr2O3 belong to this type of inclusions.
(2) Plastic inclusions This kind of inclusions have a good range during thermal deformation and extend into a strip along the deformation direction. In this category, there are sulfides and iron-manganese silicates with a low content (40% to 60%).
(3) Spherical invariant inclusions The as-cast state is spherical and remains spherical after hot working, such as SiO2 and silicate with higher SiO2 content (>70%).
(4) Semi-plastic inclusions refer to various complex aluminosilicate inclusions. The matrix aluminosilicate has plasticity and will produce plastic deformation during hot working, but the precipitated phases contained in it, such as alumina, are brittle, and remain intact or just pulled apart during processing.
4 Identification of inclusions
The early workers mainly used optical microscopes with X-ray structural analysis and chemical composition analysis, and accumulated valuable experience and rich information. In recent years, the use of electron probes to analyze inclusions in micro-domains has increased. There are currently two general methods for identifying inclusions.
4.1 The combination of metallographic method and micro-area component analysis
After selecting the pending inclusions in the metallographic observation, use the electronic exploration needle (EPMA) for micro-area component analysis or use the scanning electron microscope (SEM) with energy spectrum analysis (EDS) for component analysis. It is usually possible to determine the constituent elements and general composition of inclusions larger than 1μm in size. If you use the surface scanning of individual elements, you can get more intuitive results. Figure 4 is the surface analysis spectrum of an 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 inclusions in the bright field observation It is MnS, SiO2, and FeS, through the energy spectrometer (EDS) analysis of its composition, you can also directly get the mass fraction of each element.
4.2 Optical metallography
Under the optical microscope, observe the color, shape, size and distribution of the inclusions in the bright field; observe the inherent color and transparency of the inclusions in the dark field; observe the various optical properties of the inclusions under orthogonal polarized light to judge Type of inclusions. According to the distribution and quantity of inclusions, evaluate the corresponding level and judge its influence on the performance of steel. At present, there are many methods for testing and studying non-metallic inclusions in steel, including chemical methods, petrographic methods, metallographic methods, electron probes, and electron scanning methods.
Metallographic identification of inclusions is based on their morphology, distribution and their optical characteristics in bright field, dark field, and polarized light (Table 1), and contrast with known inclusion characteristics to determine their type. If necessary, the microhardness of inclusions or the ability to withstand corrosion by chemical reagents can be measured. See the metallographic identification procedure for non-metallic inclusions.
5 Quantitative rating of non-metallic inclusions
5.1 National standard rating
Quantitative determination is one of the routine testing items for 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 determine the quality of steel or whether it is qualified. The rating of inclusions can be carried out according to the GB/T10561-2005 standard. The sample has been carefully polished, the inclusions should be kept intact and observed under a microscope with a magnification of 100 times without corrosion. Compare the field of view with the most serious inclusions on the sample with the standard grade picture to evaluate its grade. The GB/T10561-2005 standard lists three types of inclusion levels. Oxides are one type, and sulfides are roughly subdivided into two series according to the most serious inclusions. Each series is divided into 5 levels. The higher the level, the more inclusions. If it cannot be rated as a whole number, half grade can be used. As for the alloy structural steel or tool steel used for important parts, the qualified level of non-metallic inclusions should be determined according to the requirements of the parts. For alloy structural steel, the general highest level shall not exceed 3 levels of oxides and sulfides, the sum of the two Level 5.5.
Chromium rolling bearings are classified and rated according to the GB/T18524-2002 standard. The non-metallic inclusions in the standard are divided into three categories: brittle inclusions, plastic inclusions and point-shaped invariant inclusions, each of which is divided into 0.5, 1, 1.5,..., 4, a total of eight standard levels. The content level of non-metallic inclusions in bearing steel should not be greater than that specified in Table 3.
In order to quantitatively study the influence of inclusions on performance, it is necessary to determine the statistical distribution of the size and spacing of the inclusions. When the inclusions are small, it must be carried out under an electron microscope. Quantitative measurement requires measurement of more fields of view to obtain statistical distribution. The application of automatic image analyzer can greatly accelerate the process of measurement work and obtain more accurate results.
5.2 JK standard rating
The inclusions are divided into four basic types, A, B, C and D, which are sulfide, alumina, silicate and spherical oxide respectively. Each type of inclusions can be divided into two series of fine series and coarse series according to the difference of thickness and diameter. Each inclusion is composed of five-level pictures (1~5) showing the increasing number of inclusions. When assessing the level of inclusions, a half grade is allowed. The result is expressed by the number of the worst visual field of each type of inclusion for each sample. The evaluation method of non-metallic inclusions in steel can refer to the GB/T10561-2005 standard.
5.3 ASTM rating standards
The ASTM standard rating chart is also called the modified JK chart. The classification of inclusions in the rating chart is the same as the JK rating standard chart, but the rating chart is composed of 0.5 to 2.5. It is suitable for evaluating the inclusions of high-purity steel. It is often used in products that can withstand a large amount of rolling, such as plates, pipes and wires. The result is expressed by the total number of fields of view of each type of inclusion at different levels.