| With the improvement of the steel cleanliness, mechanisms of Rare Earths (RE) in steel gradually change and the application effects are more stable and outstanding through summarizing and reviewing the development of RE application techniques in steel. Fully displaying the subtle effect of RE in clean steel, developing the RE steel which has China’s resource characteristics and transforming the resources advantage of RE into the quality advantage of steel and iron products have significant impact on the development of China’s steel industry.The rapid development of China’s railway industry puts forward higher requests for the comprehensive performance of heavy rail steel. In view of the domestic and international market demands, the second generation of RE heavy rail steel (BVRE steel) has been successfully trial-produced by Baotou Iron and Steel Company utilizing the RE resources of BaiYunEBo complex ores. The hot-rolling strength of BVRE steel reaches1080MPa. But there are some problems during industrial production of the BVRE steel, such as poor stability of microstructure, big fluctuation of properties and so on. To further improve the comprehensive performance of BVRE steel and achieve the stability control of microstructure and properties of BVRE steel, effects of RE on microstructures and properties of BVRE steel were systematically investigated. Meanwhile, effect mechanisms of vanadium and chromium in BVRE steel were also comprehensively discussed. The research results can provide the theory basis for the composition design and performance optimization of BVRE steel.In this paper, effects of RE on austenite growth, dynamic recrystallization, pearlite structure and microstructure transformation of heavy rail steel were investigated by thermal simulation experiment and metallographic analysis. It was founded that, the appropriate content of RE could improve the activation energy for austenite grain growth and refine austenite microstructure. In the present study, the activation energy for austenite grain growth was increased from556.45kJ-mol-1to646.89kJ-mol-1by RE which content was0.0145%in heavy rail steel. Meanwhile, austenite dynamic recrystallization of heavy rail steel was delayed by RE, and the lower deformation temperature and large deformation were more beneficial to play this function of RE. In addition, The CCT curve of heavy rail steel was moved to lower right by proper amount of RE, which indicated that RE delayed the pearlite transformation. Therefore, RE could refine pearlite microstructure of heavy rail steel. Both pearlte interlamellar and the size of pearlte colony firstly increased and then decreased with increasing the content of RE. In the present study, when the content of RE was0.0145%, pearlte interlamellar and the size of pearlte colony of heavy rail steel was respectively decreased by12.3%and45.1%than that of heavy rail steel without RE.Effects of RE on the strength, plasticity, impact toughness and atmospheric corrosion resistance of heavy rail steel were studied by means of tensile test, impact test, electrochemical test, fracture analysis, rust layer analysis and so on. The results showed that, the proper content of RE could improve the plasticity and impact toughness of heavy rail steel. In the present study, the elongation percentage and reduction of area of heavy rail steel which content of RE was0.0145%was respectively increased by13.7%and15.0%than that of heavy rail steel without RE; the room temperature (20℃) and low temperature (-40℃) ballistic work of heavy rail steel which content of RE was0.0145%was respectively increased by29.6%and29.4%than that of heavy rail steel without RE. In addition, the proper content of RE could improve the atmospheric corrosion resistance of heavy rail steel by increasing the corrosion potential and polarization resistance, decreasing the corrosion current density, promoting the phase transformation of rust layer from unstable phase to stable a-FeOOH and forming rust layer with good insulativity and compactability.The mechanisms of vanadium and chromium in BVRE steel were investigated. It was founded that, both vanadium and chromium could delay the pearlite transformation and refine the pearlite interlamellar. In the present study, when the content of vanadium was increased from0.052%to0.120%, the pearlite interlamellar was decreased by13.3%; and when the content of chromium was increased from0.21%to0.40%, the pearlite interlamellar was decreased by22.2%. The strength and hardness increased and plasticity decreased of BVRE steel with increasing the content of vanadium and chromium. Meanwhile, with increasing the content of chromium, atmospheric corrosion resistance of BVRE steel improved.In this paper, the microstructure and properties of BVRE steel and BNbRE steel were compared. Meanwhile, the effect differences of RE in BVRE steel and BNbRE steel were also analyzed. It was founded that, compared with BNbRE steel, BVRE steel had more refining austenite and pearlite structure, and higher strength and hardness, but the plasticity and toughness were equivalence. RE had more obvious effect on the refinement austenite and the improvement of plasticity and toughness for the BVRE steel. |
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