增氮结合控冷对高性能复相钢M-A组元的调控作用

It has been revealed in our previous works that a dual-phase microstructure containing the matrix of acicular ferrite and/or low-carbon bainite and the second phase of martensite-austenite (M-A) constituent can be prepared in a low alloyed steel with low carbon content and different levels of nitrogen content by controlled transformation of supper-cooling austenite in mid-temperature interval. By intentionally added nitrogen and properly controlled cooling，the inner structure and morphology of M-A constituent can be adjusted. Namely, the twin-type martensite/retained austenite constituent is replaced gradually by dislocation-type martensite/retained austenite constituent (DM-RA, for brevity). In the same time, the fraction of DM-RA constituent with a relatively low hardness and the amount of RA is increased obviously. As a result, a new nitrogen-microalloyed dual-phase steel with high properties combining high strength, good toughness, high strain hardening exponent and low yield-to-tensile strength ratio has been produced. The application of this type of steel will be expected in the field of earthquake- and ductile deformation- resistant design structures of long-span buildings and bridges. However, until the present one cannot understand thoroughly how and why the enhanced nitrogen combined with controlled cooling affect the inner structure and morphology of M-A constituent and further affect the tensile and impactive properties. Further works should be carried out to make the correlations clear. For this effort, by means of calculation, simulation and characterization, the emphases should be paid to understanding the role of the enhanced nitrogen combined with controlled cooling in adjustment of M-A constituent and the mechanisms of improved ductility and toughness by DM-RA constituent. The results will not only enrich the fundamentals of simultaneously enhancing the strength, ductility and toughness of microalloyed dual-phase steel, but also lay a foundation for design of a new dual-phase steel with high properties.

申请人经初步研究发现，一种添加不同氮含量的低碳低合金钢过冷到中温区转变后空冷，形成以针状铁素体和/或低碳贝氏体为基体、马氏体-奥氏体（M-A）组元为第二相的复相组织；增氮结合适当控冷，可以调控M-A组元的内部结构和形态，使它由"孪晶M-残A"变为"位错M-残A"、组元数量及残A含量增多、硬度降低，获得了高强、高韧、高应变硬化指数和低屈强比的综合性能，制备出一种新颖的"氮微合金化"高性能复相钢，在抗震设计和塑性设计的大跨度建筑和桥梁上有应用前景。目前在该复相钢中，"增氮结合控冷"与M-A组元的内部结构和形态之间、以及后者与拉伸和冲击性能之间的内在关系，还需要深入研究。本申请拟采用计算、模拟和表征等手段，侧重研究该复相钢中"增氮结合控冷"对M-A组元的调控作用及机理、"位错M-残A"组元增塑增韧的作用及机理。该研究可望丰富复相钢的微合金化和强-塑-韧化理论，为新型高性能复相钢的设计提供参考。

为研究氮含量对低碳Mo-V-Ti钢复相组织中马氏体-奥氏体（M-A）组元结构特征的影响规律及增氮对M-A组元的调控作用机理、M-A结构特征对该钢拉伸和冲击性能的影响规律及“位错M-A”增塑增韧的作用机理，本项目制备了不同氮含量（0.004～0.020wt%）试验钢，采用光学显微镜、扫描电镜、电子背散射、透射电镜、电子探针等方法进行了微观组织表征，使用Thermo-Calc/DICTR、Image Pro-plus等软件分别进行了热力学计算及组织定量分析，测试了拉伸和冲击性能，取得如下主要结果：.低碳Mo-V-Ti-N钢过冷到中温区发生γ→α＋γ'（亚稳γ）转变，存在C、N从α向γ'的共同扩散，且二者之间存在竞争关系，增氮使γ'中的C含量显著降低、N含量显著提高，使位错Ms逐渐高于孪晶Ms，导致M-A呈由孪晶M-A向位错M-A转化的趋势，从而对M-A的内部结构有调控作用。.增氮使低碳Mo-V-Ti钢针状铁素体（AF）数量增多，粒状贝氏体（GB）减少；M-A数量增多且位错M-A组元在全部M-A组元中所占的比例提高，与少量孪晶M-A相比，使组元附近基体位错增殖总量增加、边界处位错缠结数量减少，应变硬化效果增强，导致n值、均匀延伸率和抗拉强度提高、屈强比降低，从而使大量位错M-A组元具有增塑作用。.增氮使低碳Mo-V-Ti钢的复相组织由“少量AF+大量GB/孪晶M-A组元”转变为“大量AF+少量GB/位错M-A组元”，冲击断裂行为特征由“微解理形核-解理裂纹穿过平行板条扩展”转变为“大量微孔形成于M-A组元与基体的边界处、微孔长大合并成裂纹-裂纹拐折扩展，使裂纹形核功和裂纹扩展功提高，从而使大量位错M-A组元具有增韧作用。.低碳Mo-V-Ti钢两阶段控轧变形奥氏体经连续冷却可转变为准多边形铁素体（QPF）、AF、GB及M-A，增氮提高Ar3，使QPF+AF增多、GB减少、M-A组元增多。此外，典型低碳Mo-V-Ti-N钢经800～900℃终轧、740～860℃开冷、以15～30℃/s冷却、450～540℃停冷时，形成AF+GB/位错M-A组元的复相组织，具有优异的强塑韧性。适当增氮可细化低碳Mo-V-Ti钢粗晶热影响区（CGHAZ）奥氏体晶粒，促进(Ti,V)(C,N)析出，从而增强晶内块状与针状铁素体的异质形核，显著细化组织，最终使CGHAZ的强度提高、韧性改善。