Nb对高铁车轴钢相变规律及微观组织的影响

Effect of Nb on phase transformation and microstructure of high-speed axle steel

采用热力学软件Thermo-Calc、Formastor-FII型全自动相变仪、OM、HRTEM等手段研究了V微合金化、Nb-V复合微合金化车轴钢的微观组织及连续冷却相变规律。结果表明,Nb能够提高钢的A_(c1)和A_(c3)相变温度,扩大铁素体+珠光体相变区冷速范围,使贝氏体相变区向左下方移动,提高获得全马氏体的临界冷却速度。此外,Nb的添加能够使V在高温下与Nb协同析出。冷速4.2℃/s的热膨胀试样HRTEM结果发现,Nb-V微合金化车轴钢中存在大量(Nb, V)C纳米析出相,呈随机分布,平均尺寸为6.8 nm,这些析出相能够细化奥氏体晶粒,是其相变后组织硬度提高的主要原因。

The microstructure and continuous cooling transformation law of V microalloyed and Nb-V composite microalloyed axle steels were studied by means of Formastor-FII dilatometer, thermodynamic software Thermo-Calc, OM and HRTEM. The results show that the Nb element can increase the phase transition temperature of A_(c1) and A_(c3) in steel, expand the cooling rate range of F + P phase transition zone, move the bainite phase transition zone to the lower left, and promote the critical cooling speed of full martensite. In addition, the addition of Nb enables V to co-precipitate with Nb at high temperatures. HRTEM results of thermally expanded samples at a cooling rate of 4.2 ℃/s found that a large number of(Nb, V)C nano-precipitated phases existed in Nb-V microalloyed axle steel, with a random distribution and an average size of 6.8 nm. These precipitates can significantly refine austenite grains and it is the main reason for the increase in the hardness of the structure after phase transformation.