轧后热处理对珠光体钢轨相变组织及硬度的影响

Effect of post rolling heat treatment on phase transformation microstructure and hardness of pearlitic steel rail

通过在Gleeble-3500热模拟试验机上对珠光体钢轨进行双道次热压缩试验,得到试验钢在轧后不同热处理工艺下的显微组织及硬度,分析热变形后不同冷却速率、等温时间和等温温度对珠光体片层与硬度的影响及其机制。结果表明,1℃/s连冷、1℃/s欠速淬火等温转变后快冷(1℃/s-580℃-30 s)、3℃/s冷却淬火等温转变60 s后快冷(3℃/s-580℃-60 s)、5℃/s高冷速淬火620℃等温转变后快冷(5℃/s-620℃-60 s)试验钢得到珠光体+少量铁素体。而3℃/s连冷、3℃/s冷却淬火等温转变30 s后快冷(3℃/s-580℃-30 s)试验钢因等温时间不足出现了马氏体或贝氏体组织。相比于1℃/s连冷,1℃/s欠速淬火等温转变后快冷对减小珠光体的片层间距以及提高硬度有着积极的作用。延长等温时间后得到的3℃/s冷却淬火等温转变60 s后快冷(3℃/s-580℃-60 s)试验钢的珠光体层间距最细,达到73.19 nm,其片层取向多样,部分渗碳体片断裂,硬度提升幅度不大与类珠光体组织的含量增加有关。1℃/s连冷试验钢的珠光体片层最粗大,硬度最低归因于析出相NbC的过分长大以及断裂渗碳体球化。相对完整并细小的珠光体片层及残留位错的存在使5℃/s高冷速淬火620℃等温转变后快冷(5℃/s-620℃-60 s)试验钢的硬度较高,达到42.0 HRC。

Microstructure and hardness of the tested steel under different heat treatment processes after rolling were studied by means of double-pass hot compression test on Gleeble-3500 thermal simulation tester.The effects of isothermal time,different cooling rates and isothermal temperatures after hot deformation on pearlite lamella and hardness and their mechanisms were analyzed.The results show that the pearlite+a small amount of ferrite are all obtained after 1℃/s continuous cooling,rapid cooling after quenching isothermal transformation at 1℃/s(1℃/s-580℃-30 s),rapid cooling after quenching isothermal transformation for 60 s at 3℃/s(3℃/s-580℃-60 s)and rapid cooling after quenching isothermal transformation at 620℃at 5℃/s(5℃/s-620℃-60 s),respectively.However,martensite or bainite structure appears after 3℃/s continuous cooling and rapid cooling after quenching isothermal transformation for 60 s at 3℃/s(3℃/s-580℃-60 s)due to insufficient isothermal time.Compared with continuous cooling at 1℃/s,rapid cooling after quenching isothermal transformation at 1℃/s plays a positive role in reducing the lamellar spacing of pearlite and increasing the hardness.The lamellar spacing of the tested steel obtained after rapid cooling after quenching isothermal transformation for 60 s at 3℃/s(3℃/s-580℃-60 s)is the smallest,reaching 73.19 nm.The lamellar orientation is diverse,and some cementite flakes are broken.The small increase of hardness is related to the increase of pearlitic structure content.The pearlite lamellae of the tested steel after 1℃/s continuous cooling is the coarsest,and the hardness is the lowest due to the excessive growth of precipitated phase NbC and the spheroidization of fractured cementite.The relatively complete and fine pearlitic lamellae and the existence of residual dislocation make the higher hardness of the tested steel when rapid cooled after quenching isothermal transformation at 620℃at 5℃/s(5℃/s-620℃-60 s),reaching 42.0 HRC.