终冷温度对低屈强比复合析出强化钢组织与性能的影响

利用热模拟方法测定低屈强比复合析出强化钢不同速率冷却后的显微组织并绘制动态连续冷却转变曲线,然后对比了不同终冷温度下试验钢的力学性能,并利用光学显微镜、扫描电镜与透射电镜分析不同终冷温度对试验钢轧后显微组织的影响。结果表明,随冷却速度的增加,试验钢的组织由粒状贝氏体转变为板条贝氏体,未发现铁素体组织,具有高淬透性。随终冷温度由400 ℃升为450 ℃,钢中板条亚结构发生粗化,位错密度下降,但高温下合金元素快速扩散使富Cu相与Nb/Ti碳化物的数量提高,析出强化效果增强。经优化终冷温度为450 ℃,此时试验钢中粒状贝氏体比例较高,可获得高强度与低屈强比的良好匹配。

Effect of final cooling temperature on microstructure and mechanical properties of low yield ratio composite precipitation strengthening steel

By using thermo-mechanical simulation, the microstructure of a composite precipitation strengthening steel with low yield ratio under various cooling rates and the corresponding dynamic continuous cooling transformation (CCT) curves were determined. Then the effects of different final cooling temperature on mechanical properties were studied by property comparison, and that on microstructure by means of OM, SEM and TEM. The results show that the dominant microstructure changes from granular bainite into lath bainite with the increase of cooling rate and no ferrite is observed, which shows a good hardenability of the steel. As the final cooling temperature increases from 400 ℃ to 450 ℃, the lath substructure coarsens and the dislocation density decreases, while accelerated diffusion of alloy elements leads to higher number of Cu-rich particles and Nb/Ti carbides, resulting in enhanced precipitation strengthening effect. The optimized final cooling temperature is 450 ℃, at which the steel has a larger proportion of granular bainite, thus resulting a good match of high strength and low yield ratio.