近终形薄带钢连铸条件下基于氧化物冶金的组织超细化及强韧化控制机理

Although strip casting has the feature of sub-rapid solidification, the solidification microstructure is still coarse and cannot be refined by applying severe deformation due to the characteristic of near-net-shape forming, thus leading to inferior toughness and ductility. In order to solve this bottle-neck problem, in the present project, oxide metallurgy is introduced to the strip casting process with the aim of exploiting the potentiality of obtaining ultrafine inclusions during sub-rapid solidification. Then the control of strengthening and toughening of strip casting steels will be realized by ultra-refining the microstructure due to the heterogeneous nucleation induced by ultrafine inclusions. Two representative steels, i.e. difficult-to-form 6.5%Si steel without γ/α transformation and low-carbon steel with γ/α transformation, will be selected as study objects. This work will illustrate the precipitation behavior and controlling mechanism of rare earth inclusions which can induce the formation of ultrafine δ-ferrite during sub-rapid solidification, clarify the competitive behavior of preferential growth and heterogeneous nucleation for δ-ferrite grains, reveal the precipitation behavior and controlling mechanism of Ti, Mn etc oxides and their effects on inducing ultrafine intra-granular acicular ferrite. Also, the synergistic effects of sub-rapid solidification, heterogeneous nucleation as well as thermo-mechanical treatment on the microstructure evolution and mechanical properties will be systematically explained. Thus, the key theoretical points of realizing microstructure ultra-refining and strengthening and toughening based on oxide metallurgy will be established in strip casting processes. Two prototype steels, i.e. hot rolled 6.5%Si steel which meets the demands of magnetic properties and has excellent toughness and ductility to be warm/cold rolled and hot rolled low-carbon steel with a good combination of high strength, excellent toughness and ductility, will be fabricated at last.

薄带连铸的亚快速凝固组织仍很粗大，近终成形的特点却又决定了无法采用大变形来细化晶粒，带钢的强韧性及塑性很差。针对这一瓶颈，本项目提出将氧化物冶金引入薄带连铸流程，挖掘亚快速凝固过程获得超细夹杂物的潜力，利用超细夹杂物诱发异质形核实现薄带钢的组织超细化及强韧化控制。以无γ/α相变的6.5%Si钢、有γ/α相变的低碳钢两个代表性钢种为研究对象，阐明亚快速凝固过程诱发超细初生δ-铁素体的稀土夹杂物的析出行为及调控机理，弄清δ-铁素体择优生长与异质形核的竞争行为，揭示Ti、Mn等氧化物的析出行为、调控机理及诱发超细针状铁素体的机制，系统阐明亚快速凝固、异质形核、形变热处理对组织演变的协同调控作用及对材料性能的影响。据此建立薄带连铸流程基于氧化物冶金的组织超细化及强韧化控制的关键理论要点，制备出可温/冷轧且满足磁性能控制要求的高韧塑性6.5%Si钢热轧原型钢及强韧性、塑性俱佳的低碳钢热轧原型钢。

薄带连铸初始凝固组织粗大且无法通过施加大变形来实现细化，从而导致带材强塑性差。针对这一瓶颈，首先，项目组研究了薄带连铸过程非金属夹杂物的析出-长大动力学行为，计算结果表明，能够形成亚微米级超细氧化物。随后开展了低碳钢成分设计和薄带连铸实验，在铸带中获得了大量的尺寸为0.2-1.0μm的超细Ti-Al-Si-Mn-O复合氧化物且能够诱导针状铁素体形核。然后，探究了超细氧化物诱导微米级超细针状铁素体的机制。研究表明，较低的晶格错配度是促进针状铁素体异质形核的主要机制。此外，还发现了一种新的异质形核机制-贫碳区机制。探明针状铁素体和原始奥氏体为K-S关系。并且，发现同一个奥氏体转变成的针状铁素体呈规律性空间分布，所以，提出了“共线变体群”的概念，即生长方向平行的针状铁素体属于1个“共线变体群”。其次，探明了调控超细针状铁素体含量的工艺路径，阐明了主要工艺参数对针状铁素体含量、尺寸、分布等特征演化及力学性能的影响规律，建立了工艺参数-微观组织-力学性能三者间的定量关系，制备出具有针状铁素体+多边形铁素体双相组织的强塑性优良的原型钢。再次，还研究了超细针状铁素体含量对奥氏体晶粒尺寸的依赖性，证实粗大凝固组织是获得大量超细针状铁素体的重要前提条件。研究表明，引入热变形将导致针状铁素体含量减少，但针状铁素体团的尺寸却增大。此外，还发展了一种具有超细针状铁素体+马氏体双相组织的新型高强钢原型钢，屈服和抗拉强度更高，而总延伸率略微下降。另外，研究还表明，在无γ/α相变的6.5%Si钢中加入稀土，借助超细稀土夹杂物的异质形核作用也可以实现薄带连铸凝固组织的细化并大幅改善塑性，成功制备出磁性能优良的原型钢薄带。通过上述研究工作，本项目揭示了薄带连铸条件下基于氧化物冶金的组织超细化及强韧化控制机理。发表SCI论文6篇，其中，1篇被选为Steel Research International期刊封面论文，授权中国发明专利3项，培养硕士研究生3名，博士研究生2名（在读1名），一人获得2019年辽宁省优秀博士论文。

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