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Steelmaking – Refining – Control of inclusions in continuous casting steel

2024.5.10

With the rapid development of the national economy, the demand for steel has gradually increased, and the quality requirements for steel have continued to increase. Pipelines, bridges, automobiles, shipbuilding, pressure vessels and other industrial production steels require a large amount of steel, and there are also high requirements for the control of inclusion levels. Therefore, it is necessary to strengthen the control of steel purity requirements to improve the overall quality and quality of steel.

1.Basic concept of clean steel

There is no clear definition of clean steel, nor is there a scientific definition method. Generally speaking, clean steel contains a small amount of phosphorus, oxygen, nitrogen, hydrogen and sulfur impurities, so it is necessary to strengthen the reasonable control of non-metallic inclusions, such as sulfides and oxides. Generally speaking, clean steel has the following three characteristics: First, the oxygen content in steel is relatively small; second, the size and number of inclusions contained are controlled within the ideal range, and the distribution of impurities is good; third, the content of brittle inclusions is very small. Therefore, in order to improve the quality and performance of clean steel, it is necessary to have good purification technology and strengthen the introduction of advanced equipment and advanced technology. Since the early 1980s, the application of purification technology in the production activities of continuous casting, steelmaking and refining has significantly improved the cleanliness of steel. Around 2000, the number of harmful elements contained in clean steel produced in Japan accounted for only 0.005%; the clean steel produced by Baoshan Iron and Steel Co., Ltd. in my country also controlled harmful elements to about 0.008%. At present, with the increasing requirements for steel in transportation construction, national defense construction and special construction, the requirements for clean steel are also increasing, so steel companies are required to continuously improve the cleanliness of clean steel.

2.Hazard analysis of inclusions in steel

Inclusions in steel exist in the form of various non-metallic compounds such as oxides, sulfides and nitrides, resulting in uneven steel structure. In addition, due to the influence of factors such as the geometric shape, chemical composition, and physical properties of the inclusions, the mechanical properties and fatigue properties of the steel are reduced.
2.1 Alumina inclusions
Al2O3 is an oxide inclusion that has the greatest impact on steel performance in killed steel. It is a brittle non-deformable inclusion, and its thermal deformation is very different from that of the base material. Under the action of hot working stress, a large number of Al2O3 brittle inclusions deform and break, forming inclusions with sharp corners, forming a chain distribution on the base. This strong and irregular Al2O3 inclusion can scrape a crack on the base and become a stress concentration point under the action of cyclic stress.
2.2 Silicate inclusions
When the molten steel solidifies, due to the high temperature, some liquid silicates do not crystallize in time, but exist in whole or in part as supercooled liquid (i.e., glass state). When the temperature rises from 800 ℃ to 1300 ℃, the plasticity changes rapidly. The composition of silicate and aluminate inclusions is relatively complex, and they are still spherical during rolling. This kind of inclusion in low-carbon steel, especially boiling steel, will cause the wire rod to plasticize, reduce toughness, and cause strip peeling.

3.Origin and distribution of inclusions

There are two main sources of inclusions in steel: one is that they are produced during smelting, that is, the deoxidizer of ferroalloy steel is incorporated into the steel during tapping, and the secondary oxidation products of molten steel and air are mixed during pouring; the other is various factors introduced from the outside, that is, foreign inclusions, most of which are irregular in shape, large in size, and unevenly distributed.
Endogenous inclusions mainly occur under the following conditions. First, during smelting, the deoxidation products cannot be completely eliminated, or the temperature is lowered during pouring, and the deoxidation products produced after continued reaction do not have time to float up in the steel and remain, distributed in the matrix of the steel in the form of small particles, some of which are aggregated into large particles (such as Al2O3), and some are in a solid solution state in the steel (such as MnO, FeO), which are all important causes of internal defects and surface cracks in the ingot. Second, during tapping and pouring, the molten steel is exposed to the air for oxidation reaction, and oxygen combines with elements in the steel to generate secondary oxides that remain in the steel; a large number of inclusions and loose defects are generated during continuous casting, resulting in a decrease in product quality. When the molten steel solidifies, FeS, FeO, etc. are produced due to the “selective crystallization” of the molten steel, and finally precipitation occurs between the grain boundaries and dendrites.

4.Control inclusions

Impurities are usually regarded as harmful components, but in reality inclusions can be transformed into beneficial components. For example, high-sulfur steel will produce sulfide inclusions, which may reduce the strength of the steel, but for free-cutting steel, it is a brittle inclusion in the form of thin strips or spindles, with low hardness, which can greatly increase the cutting speed. If there is a small amount of calcium in the steel, the original inclusion CaO・SiO2 can be dissolved in Al2O3, or converted into 3CaO・2SiO2, which can be deposited on the surface of the tool during the cutting process, covering the surface of the tool to avoid friction between the chips and the workpiece and the front and rear surfaces of the tool.

5.Technical measures to reduce steel inclusions

In order to control and reduce inclusions, the processing measures taken during steelmaking and continuous casting operations mainly include deoxidation purification, ladle refining, filtration purification, vacuum treatment technology, and electromagnetic purification.

The presence of inclusions will have a great impact on the mechanical properties of the material. With the development of the modern economy, people have put forward higher requirements for the purity of steel products. Therefore, steel mills must accurately understand the source, distribution and main hazards of inclusions in steel, and adopt scientific treatment and purification processes to coordinate the performance and economic benefits of steel products to meet relevant use standards and promote the development of enterprises.