采用粉末冶金方法制备了一种具有双峰晶粒结构的超低碳9Cr-ODS (氧化物弥散强化)钢,通过OM、SEM、TEM、显微硬度和拉伸性能测试,研究了热处理工艺对其显微组织和力学性能的影响。结果表明,超低碳9Cr-ODS钢经正火+回火后为回火马氏体组织,具有粗、细晶分明的结构特征。细晶区的平均晶粒尺寸约为1.6 μm,粗晶区的平均晶粒尺寸约为4.3 μm。同时,基体中存在大量的位错结构,且纳米级氧化物数密度可达约10²² m^-3。不同的热处理工艺不会改变超低碳9Cr-ODS钢粗、细晶双峰晶粒结构特征。经热处理后,细晶区比粗晶区具有更高的硬度。随正火温度升高,粗、细晶区的显微硬度先上升后下降,且在1100℃正火时达到最高。正火温度一定,回火温度从700℃升高至800℃时,粗、细晶区的显微硬度先下降后上升。700和750℃回火时,组织得到回复,发生软化,温度越高硬度越低；而在800℃回火时,超低碳9Cr-ODS钢因发生部分奥氏体相变导致硬度提高。25℃拉伸实验结果与硬度的变化趋势一致,随回火温度升高,超低碳9Cr-ODS钢的强度先降低后增加,延伸率则呈现相反趋势。700℃拉伸实验结果表明,超低碳9Cr-ODS钢的强度随回火温度的升高稍有降低。结合力学性能及断口分析结果,分析了双峰晶粒结构超低碳9Cr-ODS钢的断裂机制。经1150℃、1 h正火+ 750℃、1 h回火后,超低碳9Cr-ODS钢具有最优的强塑性匹配。

Oxide dispersion strengthened (ODS) steel is a promising structural material for advanced nuclear power systems. In this study, an ultra-low carbon 9Cr-ODS steel with a bimodal grain structure was prepared using powder metallurgy, and a superior matching of strength and plasticity was expected by adjusting the soft-hard matching of the coarse-grained and fine-grained regions. The effects of heat treatment on microstructure and mechanical properties of the ultra-low carbon 9Cr-ODS steel were evaluated through OM, SEM, TEM, microhardness, and tensile tests. The results demonstrated that the ultra-low carbon 9Cr-ODS steel exhibited a tempered martensite structure after normalizing at 1050-1200^oC, and then tempering at 700 and 750^oC. Moreover, it presented the microstructure characteristics of coarse-grained and fine-grained regions, in which the average grain size of fine-grained regions was 1.6 μm and that of coarse-grained regions was 4.3 μm. The dislocation density in the ultra-low carbon 9Cr-ODS steel was very high and the number density of nano-scale oxide particles was up to about 10²² m^-3. The microhardness in fine-grained regions was higher than that in coarse-grained regions. As the normalizing temperature increased, the microhardness of the ultra-low carbon 9Cr-ODS steel first increased and then decreased. The microhardness reached the highest after normalizing at 1100^oC. When the normalizing temperature increased to 1200^oC, the microhardness decreased due to the growth of austenitic grains. Regarding the tempering temperature, the microhardness first decreased and then increased as the tempering temperature increased from 700^oC to 800^oC. Furthermore, the decrease in microhardness when tempering at 700 and 750^oC was because the microstructure was recovered and softened. The higher the tempering temperature, the lower the microhardness. However, when tempering at 800^oC, the microhardness increased significantly, mainly due to the partial austenite transformation of martensite. The tensile test results at 25^oC showed that the strength of the ultra-low carbon 9Cr-ODS steel first decreased and then increased by increasing the tempering temperature, which was consistent with the microhardness change while the opposite was observed for elongation. The tensile test results at 700^oC showed that the strength of the ultra-low carbon 9Cr-ODS steel slightly decreased by increasing the tempering temperature. Moreover, the fracture morphology was dominated by fine dimples and secondary tearing, indicating that the ultra-low carbon 9Cr-ODS steel underwent ductile fracture. Combined with the mechanical property and fracture analysis results, the ultra-low carbon 9Cr-ODS steel exhibited superior matching of strength and plasticity after normalizing at 1150^oC for 1 h and tempering at 750^oC for 1 h.