高强钢HAZ纳米复合析出相多阶段演化行为及其对强韧性的影响

The application of high strength steels is an effective approach to achieve high safety and lightweight for engineering machinery, automobile and marine equipment. However, the deterioration of the welding properties in heat affected zone (HAZ) for the conventional high strength steels with the strength above 690MPa, including welding cold crack, local embrittlement and softening and other issues, is the common problem to restrict the large-scale promotion and application. To introduce nanoscale co-precipitates into HAZ provides a new way to improve the weldability of the high strength steels. Therefore, it will have a great potential for the application in the above areas. Based on the previous research, this project proposes to optimize the chemical compositions of high strength steels, with the purpose to accelerate the nanoscale co-precipitation during multi-stage welding of continuous cooling after welding, multi-pass welding and post weld heat treatment. The evolution behavior of nanoscale co-precipitates during multi-stage welding and its effect on micrometer scale microstructure of HAZ would be studied. The control mechanisms of nanoscale co-precipitates in HAZ, as well as the coupling relationship between precipitation strengthening and other strengthening mechanisms in typical sub-regions of HAZ, would be also investigated. Finally, the chemical composition and the welding process are synergistically controlled to fully exert the beneficial effects of the precipitation strengthening in HAZ. This project will provide a new direction for improving the weldability of high strength steels efficiently, and establish a theoretical foundation for the development of a new type of high strength steel with excellent weldability and its improvement of welding process.

高强钢的应用是实现工程机械、汽车和海洋装备高安全性和轻量化的有效途径。但是，690MPa及以上级别传统高强钢的焊接热影响区（HAZ）性能恶化（焊接冷裂纹、局部脆化和软化等）是制约其大面积推广的共性难题。在HAZ中引入纳米复合析出相为改善高强钢的焊接性提供了新的途径，在上述领域具有广阔的应用前景。本项目在前期研究工作基础上，对高强钢的化学成分进行优化设计，从而加速纳米复合强化相在焊接连续冷却、多层多道焊及焊后热处理等多个阶段析出；澄清HAZ中纳米复合相在多阶段焊接过程中的演化行为及其对HAZ微米尺度显微组织的影响规律；阐明HAZ纳米复合析出相的调控机理，并明确HAZ典型亚区中纳米析出强化与其他强化方式的耦合关系；通过化学成分及焊接工艺的协同控制，使析出强化的有益效果得到充分发挥。本项目将为高效地改善高强钢的焊接性提供新的方向，为开发具有优异焊接性的新型高强钢及其焊接工艺的改进奠定理论基础。

本项目针对690MPa及以上级别传统高强钢的焊接热影响区（HAZ）性能恶化（焊接冷裂纹、局部脆化和软化等）而制约其大面积推广的共性难题，提出在HAZ中引入纳米复合析出相来改善高强钢的焊接性，重点研究高强钢HAZ纳米复合析出相多阶段演化行为及其对强韧性的影响。本项目通过热力学和动力学计算，分析了Cu、Ti和Mo的含量对纳米强化相的影响规律，设计了四种不同成分配比的高强钢，通过成分优化获得屈服强度为820MPa、-40℃冲击功为110J的良好强韧性匹配。Ti元素的添加会促进MC的析出，可以补充由于富Cu相的粗化造成的析出强化损失，进而改善HAZ的局部软化；Mo元素的添加会抑制富Cu相的析出和粗化。焊接过程中，t8/5的增加和多层多道焊会促进富Cu相在CGHAZ的析出和粗化。研究发现，焊接拘束会提高HAZ在连续冷却阶段的相变温度，促进富Cu相和MC的析出。因为HAZ在冷却阶段相变温度的提高有利于元素扩散，进而促进纳米复合析出。MC型碳化物的析出可以补充由于富Cu相粗化和溶解造成的强度损失，避免HAZ的局部软化。同时，FCC结构的MC型碳化物可以阻碍位错切过，避免了HAZ中富Cu相析出造成的局部脆化。本项目厘清了合金配比和焊接工艺要素对HAZ的纳米析出行为及强韧性的影响，这为新型易焊接高强钢的设计和优化提供了理论依据。