Influence of Finishing Cooling Temperature and Holding Time on Nanometer-size Carbide of Nb-Ti Microalloyed Steel

The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of interphase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50 — 1000 s. Interphase precipitation shows higher nano-hardness at 640 °C compared with diffusion precipitation at 600 °C, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.