Nb-Ti微合金钢第二相弥散析出行为调控机理与高温热塑性研究

With the rapid development of micro-alloyed steel production technology, the study of surface transverse crack is urgent need for breaking through the conventional control method and searching fundamental solution to improve hot ductility. The precipitation location of secondary phase particles transferred from grain boundary into matrix is contributed to inhibition of crack. With references of the pertinent research results, this present project puts forward using the theory of interface free energy to study nucleation and growth of secondary phase particles. After some factors of size evolution of secondary phase particles are obtained, they can be used to achieve the aim control of particles. The main research contents are follows. (1) thermodynamics of precipitation of secondary phase particles; (2) quantitative characterization of secondary phase particles by Mg treatment; (3) in-situ observation and mathematical analysis of size evolution of particles; (4) synergistic control and verification for hot ductility. This study is to found the inclusion type and suitable cooling rate in favor of enhancing precipitation in matrix, also it can be used to reveal the interface parameters and reaction mechanism between inclusion particles and secondary phase, which can give us more directions for the study of dispersed precipitation. The research results not only perfect the control theory of precipitation, but also supply newer thinking for inhibition of surface transverse crack.

随着微合金钢技术的进步，对铸坯裂纹的控制迫切需要突破传统“躲避”脆性区的温控策略，寻求改善高温塑性的根本性解决方案。控制第二相颗粒由晶界析出转为晶内弥散析出有助于从根源上消除铸坯裂纹缺陷。本课题借鉴国内外研究成果，系统将界面自由能理论引入第二相形核-长大过程研究，深入剖析促使第二相颗粒晶内形核及尺寸演变的影响要素，从而实现钢中第二相颗粒的灵活控制。主要研究内容包括：凝固过程中第二相粒子析出热力学研究，Mg处理对第二相颗粒的改性及定量表征，第二相粒子长大过程的原位观察与数学解析，高温塑性的协同调控与试验验证。本研究旨在确定有利于促进第二相粒子晶内形核和晶粒细化的微细夹杂类型与成分区间以及抑制粒子过度长大的冷却制度，揭示钢中微细夹杂诱导第二相粒子晶内弥散形核的界面参数和作用机制，为第二相弥散化析出指明方向。Mg处理对第二相粒子和凝固组织的双重控制可为抑制铸坯裂纹提供新的解决思路。

含Ti微合金钢角部横裂纹是其连铸生产过程中最为常见的质量缺陷。δ相转变为γ相的过程中由于结构差异伴随着大的体积收缩，导致初生坯壳厚度不均，应力集中时易产生表面裂纹，并且原始奥氏体晶粒的粗大也对铸坯质量有着不利影响，尤以包晶钢裂纹缺陷发生几率最高。.本项目对含Ti包晶钢凝固初期的液相异质形核机制和非平衡条件下包晶转变行为开展了系统研究。通过实验室Mg-Ti复合处理试验明确了MgAl2O4-TiN-δ-Fe的形核机制，并基于该形核机制对包晶钢镁处理工艺效果进行试验验证，研究结论为解决微合金钢裂纹问题提供了技术思路。镁处理钢液中形成了大量弥散分布的富镁TiN颗粒，无论从TiN与液相的润湿性角度还是TiN与固态δ相之间的晶格错配度角度而言，均证实TiN可以作为高温铁素体的有效形核质点，显著降低其临界形核过冷度，从而起到诱发凝固初期的δ相异质形核的作用。Mg-Ti复合处理铁素体不锈钢等轴晶比率由37%提高到50%，平均晶粒尺寸减小26%左右，钢液中形成了大量富含MgO/MgAl2O4核心的TiN颗粒。.基于晶格错配理论可以合理解析镁处理对铁素体组织的细化机制。HRTEM分析MgO和MgAl2O4与TiN晶格错配度分别为0.068%和5.02%，TiN与δ-Fe之间的晶格错配度为4.41%，三相之间位相关系为(200)TiN//(110)δ-Fe和(400)MgAl2O4//(200)TiN。说明TiN与钢液润湿特性促进了初始凝固液相在其表面的附着形核。高熔点氧化物Al2O3、MgO、MgAl2O4只有借助于TiN才可以起到促进铁素体相异质形核的作用。镁处理含Ti包晶钢可以通过细化原始奥氏体组织显著改善连铸坯的高温热塑性。镁处理后包晶钢原始奥氏体平均晶粒尺寸减小了18.7%左右，富镁TiN颗粒尺寸减小约41%。镁处理后铸坯的第三脆性区最小断面收缩率高于60%，断口形貌由镁处理前的沿晶断裂形式发展为韧性断裂。工业试验证实镁处理可以有效抑制含Ti包晶钢表面裂纹缺陷，铸坯角部裂纹发生率由3.79%降至0.01%，铸坯质量得到显著改善。