Transverse Cracks on the Surface of Steel Ingots and Their Effects

Abstract Steel ingot surface crack refers to a kind of internal damage mode of steel ingot which appears on the surface of steel ingot as a visible crack. It is difficult to predict the width and depth of the crack, so it often has a certain impact when using. Therefore, it is necessary to study the causes and effects of the surface crack of steel ingot in order to effectively prevent and reduce the surface crack of the steel ingot during production, storage and processing. This paper introduces the formation mechanism and influencing factors of the transverse crack of steel ingot surface in detail, and then points out that the influence of transverse crack of steel ingot on mechanical properties, plasticity, corrosion resistance and other properties are introduced preliminarily. Finally, some precautions are put forward from the perspectives of process, material selection and heat treatment before the forming of steel ingot to control and reduce the occurrence of surface crack of steel ingot. Keywords: steel ingot, surface crack, influence, prevention 1. Introduction As we all know, the steel ingot surface has a great influence on the quality of the product and the product performance. Generally, the super-thick plate and large-diameter round bar are formed by hot rolling, and the steel ingot surface plays an important role as the preformed substrate. Of course, in addition to the surface of steel ingots, there are also internal defects, such as inclusions, microsegregation and cracks, which may cause the appearance of surface cracks. This kind of crack is called surface crack of steel ingot. In the process of steel ingot production and processing, although the surface crack is easy to observe and identify, it is still difficult to predict its geometry, so it is inevitable to cause interference when used. 2. Causes and Influences of Steel Ingot Surface Crack 2.1 Formation mechanism Due to the limitation of technology, it is generally easier to form surface crack on the steel ingot surface in the casting or forging process. This kind of transverse crack is usually formed by the shrinkage stress generated inside the steel ingot due to the temperature difference between the surface and interior, or due to the sulfide segregation at the interface of the steel ingot. The shrinkage stress generated inside the steel ingot are very high, which will cause a great change in the microstructure and dimensions of the steel ingot and further lead to the formation of crack along the grain. In addition, due to the lack of complete nucleation of alloy elements and the increase of segregation coefficient at the grain boundary, the transverse crack of steel ingot is also often caused by segregation during the processing of steel ingot. 2.2 Influence on properties Under normal circumstances, if there are cracks on the surface of steel ingot, the mechanical properties and service life of the products made from it will be affected. Generally speaking, these effects mainly include the following aspects: (1) Tensile Properties. The most obvious and direct influence of surface crack on steel ingot is to reduce the tensile strength and plasticity of the steel ingot. The cracking of the steel ingot will weaken the structural strength and reduce the plasticity of the steel ingot, so it is not suitable for products that require high load-bearing capacity. (2) Corrosion Resistance. Surface crack of steel ingot will reduce its corrosion resistance. This is because the crack is like a channel, it can accelerate the penetration of air, water and corrosive gas, thus increasing the risk of corrosion. Moreover, the transverse crack of the steel ingot can accelerate the nucleation of corrosion products, which further erodes the steel ingot surface, resulting in the decrease of its service life. (3) Other Performance. Besides the tensile properties and corrosion resistance, the transverse crack of steel ingot will also affect the other properties of steel ingot, like incision and shearing resistance, impact resistance and fatigue performance. The existence of surface crack will also worsen the machinability of steel ingot. 3. Steel Ingot Surface Crack Prevention and Control In order to reduce or prevent the occurrence of surface crack of steel ingot, it can be effectively controlled from the following aspects: (1) Proper use of mould water. In the forming process of steel ingot, the temperature rising rate of steel ingot is related to the cooling rate of steel ingot, the cooling rate of steel ingot is related to the amount of mould water and the cooling speed is related to the proper selection of mould water. Therefore, using the right amount of water to cool the steel ingot can reduce the possible shrinkage stress and surface cracks. (2) Careful adjustment of stirring time. It is also important to adjust the stirring time of steel ingot carefully. For the steel ingot made of alloy steel, if its stirring time is too short, the alloy elements will have insufficient segregation, which will lead to a great extent form the stress concentration or the irregular alloy segregation, thus lead to the formation of transverse cracks in the surface of steel ingot. (3) Selecting the ordinary steel. The ordinary steel is the simplest method to reduce the surface crack. Due to its low sulfur and phosphorus content, low segregation coefficient, large hot-workable range and good mechanical properties, ordinary steel can effectively reduce the cases of transverse crack. (4) Choose the appropriate heat treatment process. Proper heat treatment can reduce the surface crack of steel ingot. Generally speaking, the quenching and tempering process can improve the microstructure and dimensional stability of steel ingot, thus reduce the transverse crack of the steel ingot surface. (5) Using machine checking. In the process of steel ingot production and storage, machine inspection can be used to check whether there is surface crack in the steel ingot in time. This kind of detection not only can identify small cracks, but also can locate the crack accurately, so as to clean up the cracked steel ingot in time, avoid unnecessary waste and ensure the product quality. 4. Conclusion Steel ingot is a common material in production, and its surface crack is the most obvious and direct external defect, which will directly affect the mechanical properties and service life of the product. As a result, it is very important to control and reduce the surface crack of steel ingot. Through the above introduction, the formation and influence of transverse crack on the surface of steel ingot are detailed and analyzed from the angles of formation mechanism and influencing factors, and some effective preventive methods are proposed in view of process, material selection and heat treatment to control and reduce the surface crack of steel ingot.