Bearing rings are annular parts of radial rolling bearings with one or several raceways, which are processed by bearing steel through forging, annealing, turning, heat treatment and other processes. Bearing steel is required to have high and uniform hardness, wear resistance and high elastic limit, so the uniformity of steel chemical composition, steel purity, content level and distribution of non-metallic inclusions, distribution and state of carbides are very strict, so bearing steel is regarded as one of the most stringent steel grades in steel production. Benxi Steel supplies a bearing factory to produce bearing rings with bearing steel GCR15 after processing individual bearing rings appear strip or dot small cracks on the surface of individual bearing rings, in order to find out the cause of bearing ring cracks, this paper carried out physical inspection analysis and **. The production process of GCR15 bearing steel is: electric furnace LF refining, VD treatment, 235 mm 265 mm continuous casting billet, heating, rolling and annealing. The processing technology of the bearing ring is: GCR15 raw material, forging, turning, heat treatment, grinding, magnetic particle detection. After rolling, GCR15 bearing steel is mainly composed of fine flake pearlite and carbide (Fe, Cr) 3C. According to the requirements of processing and use, it generally needs to be spheroidized annealing treatment or quenching + low temperature tempering treatment, and the microstructure is transformed into fine cryptocrystalline martensitic and cementite particles after heat treatment, and the internal control chemical composition of GCR15 is shown in Table 1. 
Fig. 1 Crack plane morphology: (a) Striped crack plane morphology;(b) Partial enlargement of the topography of the striped crack plane;(c) The punctate crack plane magnified morphology was used to cut the two defect sample parts, and then the metallographic sample was prepared for the defect longitudinal profile and observed under an optical microscope. No obvious abnormalities were found in the strip defect parts, but the defect parts were slightly concave, which was judged to be a long heating time of the billet, and a large number of iron oxide scales were attached to the surface before rolling, and pits were formed due to physical shedding during rollingAfter GCR15 was machined into a bearing ring, the original pit depth was not reached due to insufficient turning depth, so it left an irregular band-like defect. This defect will not extend to the substrate, if the material has no special requirements for appearance, it will not affect the normal use of the part. As shown in Fig. 2(a), one end of the strip exposes the surface of the specimen, so that the surface of the bearing ring is punctuated. The length and width of the non-metallic inclusions are about 580 m and 12 m, and the morphology of the non-metallic inclusions is observed under SEM, as shown in Fig. 2(b), which is formed by the aggregation of small granular inclusions. The results of energy spectrum analysis of the defective parts are shown in Table 2, and the chemical components are mainly Fe, O, Mg, Al, Ca, and S. The large-size strip inclusions were mainly composed of granular Mgo·Al2O3, CAO-2Al2O3 and CAS, and the most likely reason for the analysis was that the nodules in the nodules were dislodged. Referring to the composition of nodules in Shuikou, the nodules were mainly composed of Al2O3, CAO and MGO, and the contents of the two were basically the same. 
Fig. 2 Crack profile topography: (a) point profile topography;(b) The morphology of non-metallic inclusions adopts aluminum deoxidation in the bearing steel pouring process, and the phenomenon of intrusive nodule nodulation often occurs, and there are microscopic inclusion particles such as Mgo·Al2O3, Cao·2Al2O3, Cao·6Al2O3 in the bearing steel with low oxygen content, due to the high melting point and the large surface tension between the steel and the molten steel, these tiny inclusion particles are easy to be deposited on the inner wall of the nozzle during the continuous casting process, resulting in nodules at the nozzle. Smaller inclusions gradually accumulate and grow at the bottom of the concave nozzle and can easily float into the mold or adhere to the bottom of the immersion nozzle. The presence of non-deformable oxide inclusions such as Al2O3, MGO·Al2O3, and calcium-aluminate in bearing steel bars can easily lead to crack initiation [1-6]. For the ultra-low oxygen molten steel such as bearing steel, most of the Al2O3 and MGO·Al2O3 high melting point solid inclusions in the molten steel can be removed through VD vacuum treatment. However, the casting process recreates the thermodynamic conditions, which again produce solid-phase inclusions with high melting points;On the other hand, the MGO in the refractory is eroded by molten steel, which will also produce the solid inclusion MGO·Al2O3 with high melting point. In order to reduce the nodulation of the bearing nozzle, the slag of the ladle nozzle should be controlled to reduce the erosion of tundish refractories. The analysis of the defects of the two bearing rings shows that the strip-like defects on the surface of the bearing rings are the pits left by the detachment of the iron oxide scale on the surface of the bearing steel, which will not extend to the substrateThe punctate defect on the surface of the bearing ring is a crack defect caused by the germination of exogenous non-metallic inclusions, which is caused by the failure of the nodules to float in time and remain in the continuous casting billetIn the production process of bearing steel, in order to reduce nodulation at the nozzle, the slag of the ladle nozzle should be controlled to reduce the erosion of tundish refractory materials.
Article** — Metal World.