# Study on the Causes and Control Measures of Mg-Al Spinel Inclusions in U75V Heavy Rail Steel
## 1 Introduction  
Heavy rail steel is the main load-bearing component of railways. The general trend in the rapid development of railways has been toward high speed and heavy loads. Therefore, the quality requirements of heavy rail steel are becoming more and more strict, not only requiring high cleanliness but also needing to have high strength, high toughness and fatigue resistance. Inclusions in heavy rail steel are mainly divided into two categories: non-metallic inclusions and metal inclusions. Non-metallic inclusions include oxides, sulfides, silicates, etc. Metal inclusions are compounds formed by alloying elements in the melting process. The formation mechanism of inclusions in heavy rail steel is mainly affected by the composition of the raw materials, the reaction during melting and the physical and chemical changes during pouring and solidification. For example, elements such as oxygen, sulfur and phosphorus in raw materials easily react with iron in steel to produce their corresponding oxides, sulfides and phosphates. In addition, in the process of pouring and solidification, due to differences in cooling rates and crystallization rates, the formation of inclusions may also occur. Inclusions have a significant influence on the mechanical properties of heavy rail steel. On the one hand, inclusions are concentrated sources of stress, which reduce the fatigue resistance and toughness of the rail. On the other hand, inclusions also affect the hardness and strength of the rail, thus affecting its wear resistanceand brittleness. Therefore, controlling the quantity and distribution of inclusions is very important in improving the performance of heavy rail steel. In order to improve the cleanliness, strength, toughness and fatigue resistance of heavy rail steel, an aluminum deoxidation process is strictly implemented during the smelting process. However, non-metallic inclusions in steel are still the main factor affecting the cleanliness of the molten steel. Inclusions are also the key factors affecting rail performance [1; 2; 3; 4; 5; 6; 7]. In the process of producing U75V heavy rail steel, several domestic steel mills have found that there are large particles of MgO-Al2O3 spinel inclusions in damaged rail samples of heavy rail steel [8; 9; 10], which seriously affect the rail quality. MgO-Al2O3 spinel inclusions easily react with acids, alkalis and other substances, thus destroying the protective film on the steel surface and making it more susceptible to corrosion. This kind of inclusion produces stress concentration in steel, which leads to decreases in the fatigue strength of the steel and the easy formation of fatigue fractures. MgO-Al2O3 spinel inclusions form a brittle phase in steel, which reduces the plasticity and toughness of the steel and makes it prone to brittle fractures. Therefore, in order to reduce the harm of MgO-Al2O3 spinel inclusions, it is necessary to conduct in-depth research on its formation mechanism and take effective control measures to reduce its content and distribution. MgO-Al2O3 spinel inclusions have a stable, surface-centered cubic structure, with a melting point of 2135 \({}^{\circ}\)C and a high hardness. They do not easily deform during rolling, making them type-D point non-deformable inclusions. Most MgO-Al2O3 inclusions range in size from 2.0 \(\upmu\)m to 6.0 \(\upmu\)m, and exist in spherical, cubic and irregular shapes. As non-metallic inclusions in steel, MgO-Al2O3 spinel inclusions have a high melting point and poor deformation ability [11], which seriously harm the plasticity, toughness and fatigue resistance of the steel matrix [12]. As spinel inclusions do great harm to the quality of steel, many researchers have studied the formation mechanisms of such inclusions. Previous research results can be summarized into four models: (1) the carbon reduction model [13; 14]; that is, C in the refractory first reduces MgO to generate CO and Mg vapor, which then diffuse into the molten steel and react with Al2O3 inclusions in the molten steel to generate spinel inclusions.