【作者】 姜周华； 张树才； 李花兵； 朱红春； 韩培德；

【Author】 JIANG Zhouhua;ZHANG Shucai;LI Huabing;ZHU Hongchun;HAN Pei-de;School of Metallurgy,Northeastern University;College of Material Science and Engineering,Taiyuan University of Technology;

【机构】 东北大学冶金学院； 太原理工大学材料科学与工程学院；

【摘要】 超级奥氏体不锈钢耐腐蚀性能和综合力学性能优异,是高端装备制造业急需的关键材料之一。但该类钢在凝固和热加工过程面临诸多制备难题:凝固偏析与析出严重,高温塑性差,热加工极易开裂,高温氧化严重。对此,本文提出了"基于硼和稀土复合处理提升超级奥氏体不锈钢凝固质量和热加工性能"的新思路。从原子尺度揭示了"硼和稀土抑制铬、钼偏聚,减轻二次相析出,改善钝化膜稳定性"的作用机理。建立了结晶器喂带多场耦合模型,明晰了喂带过程"鞘壳生成-钢带熔化-钢带熔凝交替-钢带消失"的演变规律,揭示了喂带参数对流场和温度场的影响规律:随喂带速度提高,钢带对射流的阻碍作用增大,低温区逐渐从板坯边部向中心移动;随振荡频率提高,钢带对射流阻的碍作用先增大后不变,钢带横截面迅速减小,周围低液相率区扩大。开发了结晶器喂带热模拟装置,实现了连铸坯传热行为和凝固组织的精准模拟,揭示了"稀土夹杂促进枝晶形核,硼和稀土界面偏聚抑制枝晶生长并细化析出"以及"钢带熔化吸热、漂浮枝晶增殖促进钢液形核,细化枝晶并抑制偏析和析出"的凝固组织改善机制,掌握了喂含硼和稀土不锈钢带工艺理论,实现了等轴晶数量倍增,中心析出相含量倍减。构建了结晶器喂带铸坯表面热力耦合模型,明晰了喂带参数对表面裂纹的影响规律:铸坯窄面纵向裂纹更易沿拉坯方向开裂,提高喂带速度能抑制窄面裂纹扩展;析出相周围应力集中,析出相尺寸及表面曲度越小,越有利于抑制裂纹扩展。优化获得了有助于提高热塑性的最佳高温均质化工艺1280℃×16 h,揭示了硼和稀土对组织和成分均匀化的影响机制:稀土细化枝晶并减轻钼偏析,且与硼共同促进σ相细小弥散分布,加快了均质化过程枝晶消融、析出相回溶和钼元素均匀化,有利于降低均质化温度或缩短均质时间。阐明了"硼优先偏聚晶界形成高势垒,抑制变形过程铬、钼偏聚和二次相析出"以及"稀土细化晶粒并显著促进动态再结晶"的作用机制,实现了硼和稀土协同改善热塑性,热塑性窗口拓宽40～50℃。明晰了"稀土优先氧化提供形核核心,促进铬表面偏聚和稀土离子氧化物晶界偏聚"的联合抗氧化机制,抗高温氧化性能提升22%～45%。基于上述研究,突破了超级奥氏体不锈钢连铸和热加工过程的关键共性科学与技术问题,丰富发展了硼和稀土微合金化理论及结晶器喂不锈钢带工艺理论。研究成果对解决我国"超级奥氏体不锈钢"短缺的"卡脖子"难题、支撑高端装备制造业快速发展具有重要的战略意义。更多还原

【Abstract】 Super austenitic stainless steel exhibits excellent corrosion resistance and comprehensive mechanical properties,which is one of the key materials urgently needed for the high-end equipment manufacturing industry.However,this kind of steel faces many preparation problems during solidification and hot working:severe solidification segregation and precipitation,poor hightemperature plasticity,easy cracking during hot working,and severe high-temperature oxidation.Therefore,this paper innovatively proposed a new idea of "improving solidification quality and hot working performance of super austenitic stainless steel based on combined treatment of boron(B) and rare earth(RE)".The mechanism of "B and RE inhibit the segregation of Cr and Mo,reduce secondary phase precipitation,and improve the stability of passivation film" was revealed from the atomic scale.A multiple-field coupling model for feeding stainless steel strip in continuous casting(CC) crystallizer was established,and the evolution rule of "sheath generation-steel strip melting-alternate melting and solidification of steel strip-steel strip disappearing" during the feeding process was clarified.The influence of feeding parameters on the flow and temperature fields was revealed as follows.As the feeding speed increased,the hindering effect of the steel strip on the jet-flow increased,and the low-temperature zone gradually moved from the edge of the slab to the center.As the oscillation frequency increased,the hindering effect of the steel strip on the jet-flow first increased and then remained unchanged.The cross-sectional area of the steel strip rapidly decreased,and the surrounding low liquid phase area expanded.A thermal simulation device was developed to accurately simulate the heat transfer behavior and solidification structure of the CC slab.The solidification structure improvement mechanism of "RE-bearing inclusion promotes dendrite nucleation,B and RE interface segregation inhibits dendrite growth and refines precipitates" and "steel strip melting absorbs heat,floating dendrite proliferation promotes liquid steel nucleation,refines dendrite and inhibits segregation and precipitation" was revealed.The theory of feeding B-and RE-containing stainless steel strip was mastered,the equiaxed crystal was doubled,and the central precipitate content was halved.A thermal-mechanical coupling model for the surface of slab produced by feeding stainless steel strip in CC crystallizer was established,and the influence of feeding parameters on the slab surface cracks was revealed as follows.The longitudinal cracks on the narrow surface of the CC slab were more likely to crack along the drawing direction,and increasing the feeding speed could suppress the expansion of the narrow surface cracks.The stress concentrated around the precipitate,and the smaller the size and surface curvature of the precipitate,the more favorable it was to suppress crack propagation.The optimal high-temperature homogenization process that helped improve hot ductility was optimized and obtained as 1280 ℃ × 16 h.The influence mechanism of B and RE on the homogenization of structure and composition was revealed as follows.RE refined dendrites and reduced Mo segregation,and it together with B promoted the fine dispersion distribution of σ phase,accelerating dendrite melting,precipitate dissolution,and uniform distribution of Mo element during the homogenization process,which was beneficial for reducing the homogenization temperature or shortening the homogenization time.The mechanism of "B preferentially segregates to form high potential barrier,inhibiting segregation of Cr and Mo and secondary phase precipitation during deformation" and "RE refines grains and significantly promotes dynamic recrystallization" was clarified.B and RE synergistically improves the hot ductility,and widens the hot working window by 40～50℃.The joint antioxidant mechanism of "RE preferential oxidation provides nucleation core,RE promotes Cr surface segregation and RE ion segregates at oxide grain boundary " was clarified.Accordingly,the high-temperature oxidation resistance was improved by 22%～45%.Based on the above research,the key common scientific and technical problems in the CC and hot working processes of super austenitic stainless steel were broken through,and the theories of B and RE microalloying as well as feeding stainless steel strip into crystallizer were enriched and developed.The research results have important strategic significance for solving the "bottle-neck" problem of "super austenitic stainless steel" shortage in China and supporting the rapid development of high-end equipment manufacturing industry.更多还原