基于硼优先晶界偏聚抑制钢中低熔点元素铋的晶界脆化

Grain boundary embrittlement is an important factor causing cracks in continuous casting straightening and rolling in steel production process, which seriously affects the smooth production of steel and the quality of steel. How to effectively suppress the hazards of grain boundary embrittlement of harmful elements in steel has become a science difficult problem in the production of steel materials..This project aims to eliminate the grain boundary embrittlement hazards of low melting point element bismuth in steel. A new idea of adding trace boron and titanium to the bismuth containing steel is put forward in this project. It will improve the thermal plasticity of steel essentially so as to avoid the high temperature deformation crack by suppressing bismuth grain boundary segregation by promoting boron preferential grain boundary segregation based on the protective effect of titanium on boron. Firstly, the grain boundary segregation of bismuth during the high temperature deformation process will be studied, the mechanism of grain boundary embrittlement of bismuth in micro scale will be revealed. Secondly, the mechanism of preferential grain boundary segregation of boron promoting by titanium will be calrified. On this basis，the effects of adding composite boron + titanium on the grain boundary embrittlement behavior of bismuth will be analyzed. The relevance among bismuth grain boundary segregation, boron and titanium content, thermal deformation parameters and material characteristic parameters will be established. Finally, the control mechanism of the grain boundary embrittlement behavior of bismuth suppressing by composite boron + titanium will be clarified. These research results will provide important support for the industrial production of high performance bismuth free cutting steel, and also provide a new theoretical basis and direction for eliminating the hazards of grain boundary embrittlement of residual elements in steel, such as arsenic and antimony.

晶界脆化是引起钢材生产过程中连铸矫直和轧制裂纹产生的重要因素，严重影响钢铁冶金生产顺行及钢材质量。如何有效抑制钢中有害元素的晶界脆化危害，已成为钢铁材料生产面临的一个科学难题。.本项目旨在消除钢中低熔点元素铋的晶界脆化危害，提出在钢中复合加入微量硼和钛元素，基于钛对硼的保护作用，促使硼原子优先偏聚到晶界，抑制钢中铋的晶界偏聚，从本质上改善钢的热塑性以避免高温变形裂纹的产生这一新思路。首先基于高温变形过程中铋的晶界偏聚行为研究，微观尺度揭示铋的晶界脆化机理；其次阐明钛促进硼优先晶界偏聚的调控机制；在此基础上，研究复合添加硼+钛对铋晶界脆化行为的影响，建立铋晶界偏聚行为与硼、钛元素含量、高温变形参数、材料特征参量的内在关联，最终揭示复合硼+钛抑制铋晶界脆化行为的调控机制。研究成果可为高性能铋易切削钢的工业化生产提供重要支撑，同时也可为消除钢中残余元素砷、锑等的晶界脆化危害提供新的研究方向。

针对低熔点元素铋在钢材连铸和轧制生产过程易导致晶界脆化，诱发高温变形裂纹的共性难题，项目提出了协同硼、钛元素提高含铋钢热塑性的调控策略，基于钛协同作用下促使硼优先晶界偏聚的作用机制，抑制铋的晶界偏聚。首先开展铋的晶界偏聚行为及脆化机理研究；结果表明，铋显著降低钢850～1000℃温度范围的热塑性，热塑性的降低主要与铋的晶界偏聚有关；铋易于在奥氏体晶界偏聚，在晶界上以Bi膜、单质Bi相、Bi-MnS复合相的形式存在，在高温变形过程中以液态形式存在而脆化晶界；铋在晶界上还可以多层铋原子层的形式偏聚，形成Bi-Bi金属键，弱化晶界强度。其次开展钛促进硼优先晶界偏聚的调控机制研究，当实验钢中单独添加硼时，硼与钢中氮结合形成BN，对热塑性产生不利影响；硼、钛协同作用下可显著提高含铋钢的热塑性，当钛含量达到0.024%及以上时热塑性较好；BN和TiN具有明显的竞相析出关系，足够高含量的钛能够促使TiN优先析出，有效抑制BN析出，以增加对热塑性有利的固溶硼含量；热力学计算结果显示，钢中BN析出量随钛含量的增加而减小，当硼含量为0.0050%、氮含量为0.0040%时，钛元素达到一定含量后（0.045%）BN几乎不析出。在上述基础上，开展复合硼+钛抑制铋晶界脆化行为的调控机制研究，在较低、较高铋含量或氮含量的实验钢中，复合硼+钛均可有效提高热塑性，低碳高硫铋易切削钢和含铋奥氏体不锈钢的热塑性均可基本上达到不含铋时的水平；复合硼+钛可促进硼优先在奥氏体晶界发生偏聚，降低铋晶界偏聚的危害，此时铋主要以单质Bi相、Bi-TiN复合相、Bi-MnS复合相和Bi-TiN-MnS复合相的形式存在，在晶内分布；复合硼+钛有利于高温变形过程中低ΣCSL晶界占比的增加、动态再结晶的发生、晶界变形协调性的提高，有效抑制铋的晶界脆化，提高含铋钢的高温变形能力。研究成果可为高性能铋易切削钢的工业化生产提供重要支撑，同时也可为消除钢中残余元素砷、锑等的晶界脆化危害提供重要借鉴。