采用SEM, EBSD, HRTEM和物理化学相分析等技术分别对0.1%Nb和0.1%Nb-0.19%Mo微合金低碳热轧钢进行了微观组织形貌、钢中析出相及强化机理的观测和分析. 结果表明, 与Nb钢相比, Nb-Mo钢的组织较为细小, 组织中小角度晶界密度也较高, 且Mo的添加使得Nb的析出率升高, 尺寸在10 nm以下的纳米级MC型析出相(Nb, Mo)C含量较高, 这种纳米级析出相(Nb, Mo)C具有较低的熟化速率, 不易粗化, 因此具有较高的沉淀强化增量, 这也是Nb-Mo钢强度高于Nb钢的主要原因.
Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide precipitated in Nb-Mo mircoalloyed as-rolled steel are rarely reported. Therefore, in the present study, the strengthening mechanism, microstructure and the precipitate characteristics of Nb and Nb-Mo microalloyed steels produced by thermo mechanical control process (TMCP) were comparatively investigated by means of SEM, EBSD, HRTEM and physical and chemical phase analysis, in order to systematically study the synergistic effect of Nb-Mo addition on the strength of as-rolled steel. The results shows that the microstructure is finer and the density of low-angle grain boundaries is higher in Nb-Mo microalloyed steel compared with that of in the Nb microalloyed steel. What's more, the Mo addition could increase the precipitation ratio of Nb, and the amount of the MC-type carbide with nanometer size in Nb-Mo microalloyed steel is evidently larger than that of in Nb microalloyed steel. Those MC-type carbide were identified as synthetic carbide (Nb, Mo)C, exhibiting low coarsening rate than that of NbC precipitated in Nb microalloyed steel, which thus contributed to a higher precipitation hardening. This is main reason of the difference in strength between Nb and Nb-Mo microalloyed steel.
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