超高强钢焊接熔敷金属联合贝氏体形成及转变机制研究

As an alternative to the high strength low alloy steel, the ultra-high strength steel has been gradually applied to the key engineering structure fields. However, the development and application of ultra-high strength steel is limited because of the lack of the corresponding welding consumables with equal strength and toughness match. The main reason is that the strengthening and toughening theory on deposited metals is still not perfect. The applicant found the new coalesced bainite (CB) constituent and thus has the important influence on the strength and toughness of deposited metal of ultra-high strength steel, which proved the discovery of Prof. Bhadeshia’s research team. Coarse CB is extremely detrimental to low temperature toughness. Therefore, this project focuses on the 800-1000MPa grade ultra-high strength steel and then the formation and transformation mechanism of CB in weld deposited metal will be studied. Based on the Design-Expert optimal designation, the effects of alloy elements on the formation of CB and mechanical properties will be investigated. The relationship of alloy composition, microstructure and properties will be obtained. Moreover, based on in-situ observation and thermal simulation, the formation and evolution of CB constituent will be investigated during the crystallization, solidification and solid-state phase transformation of welding pool. The nucleation and growth of CB will be analyzed and the CCT curve will be also drawn. Finally, the formation of CB will be effectively adjusted by optimizing alloy elements, welding processes, external energy and heat treatment. The transformation mechanism of CB will be discussed. The design theory on the interlaced multiphase microstructure of weld deposited metal of ultra-high strength steel will be improved. This project will provide theoretical basis and experiment instruction to improve the strength and toughness on weld deposited metals. It is of significance to promote the development of basic theory of welding metallurgy and enlarge the application of ultra-high strength steel.

超高强钢正逐步取代低合金高强钢应用于重要工程结构，但缺乏等强韧性匹配焊材使其发展和应用受到限制，究其原因是现有焊接熔敷金属强韧化理论不完善。申请人在超高强钢焊接中发现了对强韧性有重要影响的联合贝氏体（CB）新组织，证实了Bhadeshia教授团队发现，粗大CB会严重恶化低温韧性。本项目聚焦800~1000MPa级超高强钢，开展焊接熔敷金属CB形成及转变机制系统研究：基于Design-Expert优化设计研究合金元素交互作用对CB产生及性能影响规律，获得成分-组织-性能关系；基于原位观察和热力模拟研究熔池结晶凝固和固态相变过程中CB形成及演变规律，分析形核和长大过程并绘制CCT曲线；结合焊接工艺、外加能量及热处理对CB形成进行有效调控，阐明CB转变机制，完善复相分割微观组织设计理论。本项目将为焊接熔敷金属强韧化提供理论依据和设计指导，对促进焊接冶金基础理论发展和扩大超高强钢应用具有重要意义。

超高强钢具有超细晶、高洁净度、高均匀性和高强韧性等特性，正逐步取代低合金高强钢应用于汽车、舰船、石油管道、海洋平台、航空航天等领域。然而由于焊接材料发展的滞后，超高强钢缺乏与之相配套的等强韧性焊接材料，使其发展和应用受到限制，究其原因是现有焊接熔敷金属强韧化理论不完善。本项目聚焦800-1000MPa级超高强钢，开展了焊接熔敷金属微观组织形成及转变机制系统研究：基于Design-Expert优化设计研究了C-Mn-Si、Cr-Mo-Ni、Zr-Ti-Ce合金元素交互作用对微观组织产生及性能影响规律，获得成分-组织-性能关系，设计出了与超高强钢等强韧性匹配系列专用焊丝，焊接熔敷金属的抗拉强度915MPa，屈服强度800MPa，断后伸长率17.0%，-40°C冲击吸收能量平均值97J，具有最佳的强韧性匹配。通过VL2000DX高温共聚焦激光扫描显微镜原位观察和Gleeble3800热力模拟试验研究了超高强钢焊接熔敷金属微观组织中奥氏体、铁素体、贝氏体及马氏体的生长行为及夹杂物形核的影响，并结合成分-组织-性能进行分析，添加1%Pr6O11会促进夹杂物细化和球化，奥氏体晶粒细化，针状铁素体优先在奥氏体晶界内形核长大，将奥氏体晶粒分割，抑制上贝氏体和马氏体的长大，形成复相分割组织，冲击韧性最好；t8/5为3-12s时，微观组织由蜕化上贝氏体、粒状贝氏体和针状铁素体组成，奥氏体晶粒内部形成复相分割结构。结合焊接工艺和外加能量条件对超高强钢焊接熔敷金属微观组织形成进行了有效调控，90%Ar+10%CO2保护气体组成对熔敷金属获得最佳强韧性匹配和低分散度最为有利，超声振动可使熔敷金属的微观组织复相分割程度增大，晶粒细化，冲击韧性提高且分散度降低，接触式超声振动细化晶粒提高冲击韧性效果更明显。本项目将为焊接熔敷金属强韧化提供理论依据和设计指导，对促进焊接冶金基础理论发展和扩大超高强钢应用具有重要意义。