低碳微合金钢中超细组织的热稳定性分析

Thermo-stability of Ultra-fine Microstructures in Low Carbon Microalloyed Steel

研究了经过新开发的弛豫-析出控制相变RPC技术得到的钢板在650℃等温回火过程中组织与性能的演变，并与经过930℃保温1h后再淬火（RQ）工艺处理后的钢板进行了对比。回火前两种钢板均为贝氏体和少量马氏体的复合组织，经过RPC处理后的钢板回火0．5h后，金相尺度下的组织没有明显变化，但硬度下降较快，在1～7h的回火过程中组织中局部区域出现板条合并现象，此阶段硬度值变化不明显，7h之后某些区域组织的板条特征趋于消失，出现了少量的多边形铁素体，硬度开始明显下降，回火20h后，大约一半的组织转化成了多边形铁素体。而经过RQ处理后的钢板回火前硬度虽然较低，但回火过程中软化速度极快，板条组织很快消失，最终获得全部的多边形铁素体组织。结果表明超细组织的热稳定性取决于其加热历史。

The evolution of microstructures and hardness of a steel plate during tempering at 650℃ has been investigated. The steel plate was manufactured by a recently developed RPC technique, which was compared with the one reheated to 930℃ and held for 1h before quenching, The microstructures of both of RPC steel and RQ steel are constituted by ultrafine bainite along with little martensite before tempering. No obvious change was detected by means of optical microscopy in RPC steel tempering for half an hour, while accompanied by obvious drop of hardness. Bainite laths started to coalesce in some regions, but the sample hardness kept nearly constant during tempering from 1 h to 7 h. With further tempering, polygonal ferrite was formed in local regions while hardness decreased dramatically. Until tempering for 20 h, only about half of bainite laths transformed into ferrite. Despite the RQ steels have lower original hardness, the samples softened faster during tempering. Boundaries of bainite laths of RQ steel disappeared quickly and finally transformed into polygonal ferrite completely. These results indicate that thermo - stability of ultra - fine microstructures is tightly depended on their thermo - history.