Study On Formation Mechanism And Control Of Pits During Inspection Process In GCr15 Bearing Steel

Significant number of micro-size pits larger than 20 μm were observed on the cross-section of GCr15 hot-rolled bearing steel wire rods which produced in some plants of our country after they were deep-etched by hot acid solution according to a Chinese standard called GB/T 18254-2002. It was reported that the machining property and fatigue life of the GCrl5 bearing steel with significant number of pits after etching were very poor. In the current study, the distribution of alloy element and corrosion behavior of GCr15 bearing steel during process of casting and rolling were investigated. Then the formation mechanism of the pits was revealed. Accordingly, laboratory experiments were designed and carried out. Applying the results, the pits could be remarkably reduced in the industrial experiments. The following conclusions were obtained.The solidification structure of bearing steel billet under mold electromagnetic stirring (M-EMS) was observed. It was found that the area percentage of equiaxed zone was about 16.8% and the center of equiaxed zone was away from the middle center of the billet due to the cooling intensity having not the identical value at the symmetry side. The deflection angle of columnar dendrite by the rotational flow recirculation on the cross-section of steel billets was analyzed. It was largest at the middle width and the middle thickness of the billet and smallest at the corner of the billet and it reached maximum values from 18-23 deg. for a velocity from 0.35 to 0.40 m/s. Any further increase in velocity had minor changes on the deflection angle.A mathematical model was developed to analyze the relationship between the stirring intensity of EM field and characteristic parameters of solidification structure. The model was validated using experimental data on magnetic intensity. Higher stirring intensity in the molten steel induced by M-EMS increased area percentage of equiaxed zone and it reached a maximum value and then remained unchanged, a exponential relationship between them. The value of PDAS increased firstly and then decreased, a parabolic relation between them. The value of SDAS increased and the cooling intensity decreased as the stirring intensity increased, a positive liner relation between SDAS and stirring intensity meanwhile a negative liner relation between SDAS and cooling intensity. Higher stirring intensity also increased the deflection angle of columnar dendrite and it also reached a maximum value and then kept same, a exponential relationship between them.The precipitated phases of GCr15 bearing steel billet were investigated using OM, SEM-EDS and EPMA, shows that alloy element segregated to the zone between the dendrite arms and combined with C to form aliquation carbides. The quantity of aliquation carbide increased as the SDAS increased. More aliquation carbides were found in the central zone with serious center segregation of the billet. From the side to the center of the billet, the size and number density of aliquation carbide both became larger.The deep etching behavior of high carbon-chromium bearing steel was investigated from continuous casting to each step of the rolling process. The Cr-rich carbide in the high Cr zone was generated due to its strong and positive segregation at the solidifying forefront in the remained liquid steel between dendrite arms. The Cr-rich carbide between the dendrite arms could not be completely eliminated before the billet was rolled into wire rods and subsequently into band. The pits originally occurred and gradually extended in Cr-rich carbide bands under severe acid conditions. So, the pit of the wire rod was stemmed from the formation of interdendritic Cr-rich carbides. With the increase in corrosion time, the pearl ite matrix with lower Cr than the network carbide surrounding the grain boundary was etched off and the corrosion pits were gradually enlarged. With further increase in corrosion time, part of Cr-carbide skeletons at grain boundaries were detached off and several corrosion pits were combined to generate big ones.Laboratory experiments were performed by adjusting the high temperature diffusion of hot-rolled wire rods and billet to reduce the pits. It was found that the high temperature diffusion of billets could reduce the dendritic segregation of Cr and improve the quality of the final rolled rod. Applying the results, the time of heat treatment of billet produced by the current plant should be longer than 480 minutes at 1240℃. The high-temperature diffusion and the normalizing at 920℃ of hot-rolled wires could also significantly reduce the pits on the cross-section of the rod. Industrial practices also showed that adjusting the diffusion parameters of rolling process and increasing the soaking temperature and time could improve the product quality significantly.