超大线能量焊接用钢多相粒子诱导热影响区多重组织细化机理研究

High heat input welding steel is increasingly applied in engineering fields. However, under super high heat input larger than 400kJ/cm, single microstructural control mechanism is invalid for grain refinement of coarse grained heat affected zone (CGHAZ). Impact toughness decreases dramatically, which becomes an imperative problem to be solved. The present project proposes “Ti–RE composite pre-deoxidation → Mg moderate final deoxidation + BN precipitation” control route to realize multiple microstructure refinement and toughness improvement of CGHAZ by introducing multi-type and multi-scale precipitation particles into steel. Based on this route, the present project will carry out research as follows. Different particles will be obtained through control of deoxidation and alloying during steel melting. The evolving rule of particles in melting–rolling–welding process flow will be analyzed. Influence of precipitation particles on transformation behaviors will be investigated through thermo-mechanical simulation and other methods . Austenite grain pinning and particle-induced ferrite nucleation mechanisms will be studied. Synergistic design of chemical composition and precipitation particles will be carried out aiming at 490MPa grade construction steel. 500~1000kJ/cm welding evaluation will be performed. Through this project research, multi-phase precipitation behaviors under melting–welding full process flow and control mechanism will be clarified. CGHAZ multiple microstructure refinement mechanism will be clarified. The study will break the bottleneck restricting super high heat input welding steel and provide theory reference for its development and application.

大线能量焊接用钢在工程领域的应用不断增加，然而400kJ/cm以上超大线能量下，单一组织调控机制难以实现热影响区组织有效细化，冲击韧性大幅下降，成为亟待解决的难题。本项目提出“Ti–RE复合预脱氧→Mg适度终脱氧+BN析出”调控路线，在钢中引入多类型、多尺度的析出相粒子实现热影响区多重组织细化，提高冲击韧性。基于这一路线开展本项目研究，通过脱氧合金化控制引入不同类型析出相粒子，分析冶炼－轧制－焊接流程中多相粒子的析出规律，采用热模拟等方法研究析出相对组织转变行为的影响，考察焊接热循环过程中奥氏体晶粒钉扎和铁素体相变诱导形核机制，针对490MPa级建筑用钢进行合金成分和析出相的协同设计，并进行500~1000kJ/cm焊接性能评价。通过本项目研究，将明确冶炼－焊接全流程条件下多相粒子的析出行为和控制机理，阐明热影响区多重组织细化机制，突破超大线能量钢制约瓶颈，为其研发和应用提供理论参考。

本项目针对超大线能量下钢材焊接粗晶热影响区（CGHAZ）组织细化和韧化难题，提出“Ti–RE复合脱氧→Mg终脱氧+BN复合析出”调控路线，引入多类型、多尺度析出相粒子实现CGHAZ多重组织细化和冲击韧性提高。对氮氧化物的析出行为热力学计算结果表明，AlN、BN在较低温度下溶解而无法发挥钉扎作用，TiN的液析行为限制了其细化奥氏体效果，Ti、Ce、Mg在钢液中形成高热稳定性固相氧化物，与氮化物复合添加时在钢液凝固及冷却过程依次析出各元素的氧、氮化物，为复合第二相的调控创造了条件。对CGHAZ显微组元的脆化机制研究表明，晶内针状铁素体可有效阻碍裂纹扩展，但片层状晶界铁素体构成裂纹传播的快速通道，导致冲击韧性及稳定性降低，实现超大线能量焊接需同时有效钉扎奥氏体、促进晶内铁素体和等轴晶界铁素体转变。对系列含硼实验钢的研究表明，固溶N元素导致CGHAZ中贝氏体和M/A岛含量增加，对焊接性能具有明显不利影响。在适量的氮硼复合添加条件下，一方面抑制原奥氏体晶粒粗化，另一方面促进晶界铁素体和针状铁素体形成以及M/A岛的分解。对Ti-Mg实验钢研究表明，Mg处理使粗大SiO2型夹杂物转化为细小球状Ti-Mg-O型夹杂物，减轻了应力集中，消除了夹杂物对基体性能的不利影响。针对Ti–RE(Ce)→Mg+BN工艺的研究表明，Ti–Ce脱氧可增加并细化一次脱氧产物，为Mg终脱氧提供充足氧源，获得氧化物的微细弥散分布。进一步，BN的复合析出显著增强第二相诱导晶内铁素体形核能力，并促进等轴晶界铁素体转变，实现组织的均匀细化，实验钢在720kJ/cm超大线能量下焊接性能显著提升。本项目研究创新了超大线能量焊接用钢成分设计路线，明确了氧氮化物多相粒子的析出行为和控制机理，阐明了CGHAZ多重组织细化和韧化机制，突破超大线能量钢性能提升制约瓶颈，为高性能先进产品的生产研发提供有力理论支撑和指导。