Study On The Effect Of Residual Elements In Gear Steel

In view of the high content of residual elements like tin in20CrMnTi gear steel produced in Xining Special Steel Co. Ltd., which can cause cracks, surface defect in productive process and the unstability of microstructure and hardenability of product. The effect of residual elements like tin in20CrMnTi steel was systematic studied in the paper. The effect of tin on the structure transformation and properties such as mechanical and impact properties, hardenability and fatigue property of20CrMnTi steel was first studied systematically. Meanwhile, the harm mechanism of residual elements like tin on the hot ductility of20CrMnTi steel, the improvement method of hot ductility, high-temperature oxidation behavior and the structure transformation and hardenability of20CrMnTi steel, which the hot ductility was improved, have been also investigated. Main results were listed as follows:In this paper, the critical cooling rate of martensitic transformation decreases and the pearlite region becomes wide as the increase of tin content.0.049%tin causes the Ac3temperature rise by15℃and Ms decrease by13℃. To obtain the ferrite and pearlite microstructures, the cooling rate should be control in0.2～1℃/s. When tin content is less than0.049%, with the increase of the tin content, tensile and yield strength of20CrMnTi steel increases and there is no significant effect on the reduction of area (RA) and elongation and the tensile fracture is ductile fracture; however, the impact energy decreases as the tin content increases and the impact fracture is quasi-cleavage fracture; there is also no significant effect on the hardenability and fatigue property.The main harm mechanisms of tin and/or copper on the hot ductility of20CrMnTi steel were①tin segregating to grain boundaries, reducing the surface energy of the grain boundary, weakening the intergranular cohesion, accelerating the nucleation and growth of the grain boundary voids and impeding the grain boundary migration and dynamic recrystallization;②copper sulfide precipitating at grain boundaries which could encourages intergranular failure by promoting voids formation and making it easier for cracks to link up as well as copper retarding the dynamic recrystallization. With the increase in tin content/copper equivalent, the hot ductility of20CrMnTi steel reduced significantly and the hot brittle range becomes wide, in which the hot ductility first drops and then rises, and there exists a ductility trough at750℃. The formation of the ductility trough is due to the proeutectoid ferrite precipitation surrounding the austenite boundaries; because the ferrite yield strengthen is relatively lower than that of austenite and thus it is easy to form stress concentration on the ferrite film and then deteriorates the hot ductility. Moreover, the critical tin content and copper equivalent, under which the hot ductility is not deteriorated significantly, in20CrMnTi steel are0.021%and0.15respectively.The hot ductility of20CrMnTi steel with tin was improved significantly by the addition of boron and rare earth yttrium. With the increase in B/Y content, the hot brittle range becomes narrow, the ductility trough shifts to a lower temperature and becomes shallow and the hot ductility increased gradually. The main mechanisms were the suppression of tin segregation to the austenite boundaries by the addition of boron and yttrium, increasing the intergranular cohesion, retarding the austenite/ferrite deformation, which presumably avoids ferrite film formation at the austenite grain boundaries, increasing the resistance to grain boundary sliding during the straightening operation and accelerating the onset of dynamic recrystallization (DRX) as well as suppressing the sulfur segregation to grain boundaries. Moreover, boron could promote the intragranular nucleation of ferrite, soften the austenite and make the austenite grain interior more deformable and hence it can improve the hot ductility significantly. In this study, adding92ppmB/0.05%Y can obtain the best effect in improving the hot ductility of20CrMnTi steel with tin.According to the study of high-temperature oxidation behavior, when20CrMnTi steel with tin was oxidized in air at1150℃and1250℃for1h, there is no Sn-rich phase at the oxide/steel interface as the large tin solubility in steel and diffusion coefficient. When20CrMnTi steel with copper and tin was oxidized in air at1150℃for1h, there is Cu-rich phase at the interface because the low diffusion coefficient for copper and tin can reduce copper solubility in austenite; while oxidized at1250℃, there is no Cu-rich phase at the interface as the enrichment of Cu-rich phase is less than the consumption. The oxide layer is divided into three layers, in which the outer is Fe2O3, the middle is Fe3O4and the inner is FeO. From the study of effect of silicon on the high-temperature oxidation behavior of steel containing copper and tin, it can be found that silicon could reduce the rate of oxidation, which could decrease the amount of Cu-rich liquid phase at the oxide/steel interface and thus reduce the penetration into grain boundaries; moreover, silicon could also increase the internal oxidation and the interface roughness which could migrate the Cu-rich liquid phase into the oxide layer and thus decrease the amount of Cu-rich liquid phase further. Therefore, the hot shortness was suppressed. Adding92ppm B into20CrMnTi steel with tin, the Ac3is844℃. So, the heating temperature before quenching should choose the temperature range of874℃and894℃. The ferrite and pearlite microstructures could be obtained when the cooling rate (CR) was controlled in the range of0.2and1℃/s; when the CR is3℃/s, the microstructures were granular bainite and martensite as well as very small amount of ferrite; when the CR is more than10℃/s, the microstructure in steel is martensite only. Boron can increase the hardenability of20CrMnTi steel with tin, moreover, when the boron content is increased from15ppm to90ppm, the hardenability is nearly identical.