Investigation On Alloying And Mechanical Properties Of Medium-High Carbon Pearlitic Steel For Wheel

The strength and toughness of medium and high-carbon pearlitic steels depend mainly on the fraction and substructure of ferrite and pearlite. Refining austenite grain size is an effective way to improve strength and toughness in pearlitic steels. It is an important means of improving strength and toughness for wheel steels by V microalloying and Si alloying without significant change in processing conditions.The effects of V and Si on microstructure and mechanical properties in medium carbon pearlitic steels for high speed wheel were systematically studied by combining Thermo-Calc and experimental research (OM, SEM, TEM, EBSD, XRD and tensile and impact tests). The results show that The austenite grain size (23.9μm-14.9μm), pearlite colony (9.8μm-5.8μm), pearlite block(14.6μm-7.6μm) and interlamellar spacing (0.172μm-0.140μm) were significantly refined by increasing V content from 0.03 wt% to 0.12 wt%, which also led to an increase in volume fraction of proeutectoid ferrite (4.3%-21.4%). With the increasing of V content, the yield strength at room temperature and the impact toughness at -20℃ were enhanced due to precipitation strengthening and grain refinement effects of VC. However, the tensile strength at room temperature was decreased due to the increasing of soft phase, i.e., proeutectoid ferrite. The increase of Si content resulted in the great decrease of proeutectoid ferrite (19.5%-12.2%) and the significant refinement of pearlite interlamellar spacing (0.159μm-0.147μm) but the slight refinement of austenite grain size and pearlite colony/block size. It is also promoted the VC precipitation with the addition of Si, but had only a little influence. The yield strength (485 MPa-531 MPa) and tensile strength (813 MPa-907 MPa) were enhanced mainly by the effects of solid strengthening and pearlite interlamellar spacing due to Si addition. The balance of strength and toughness in medium carbon pearlitic steels could be effectively optimized by the combination of medium V microalloying (0.07-0.08 wt%) and relatively high Si alloying (0.8-0.9 wt%).The influence of V on microstructure and mechanical properties in high carbon pearlitic wheel steels was investigated by the means of OM and SEM. By increasing V content from 0.04 wt% to 0.13 wt%, the austenite grain size (22.2μm-11.2μm) was refined significantly, but the pearlite interlamellar spacing (0.083μm-0.105μm) was increased. The formation of a continuous grain boundary cementite network was suppressed and a small amount of grain boundary ferrite was formed in the hypereutectoid steel with the addition of V. The yield strength and tensile strength were decreased due to the increase of interlamellar spacing of pearlite. The impact toughness was improved due to the refinement of austenite grain size.To clarify the microstructural unit controlling toughness in pearlitic steels, the correspondence between pearlite colony/block size and cleavage facet size as well as the cleavage crack propagation path were investigated mainly by using SEM and EBSD. The pearlite block and cleavage facet sizes decreased with the refinement of prior austenite grain size, whereas the pearlite colony size only changed slightly. Moreover, pearlite block size corresponds well to cleavage facet size. The cleavage crack can largely deflect at block boundaries in most cases. The ferrite/cementite interfaces and pearlite colony boundaries, however, cannot obstruct crack propagation effectively. Therefore, refining pearlite block size and improving the homogeneity play an important role on the improvement of toughness.