Kirbiš Peter, Anžel Ivan, Rudolf Rebeka, Brunčko Mihael
Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia.
SIJ Metal Ravne d.o.o., Koroška cesta 14, SI-2390 Ravne na Koroškem, Slovenia.
Materials (Basel). 2020 Mar 9;13(5):1220. doi: 10.3390/ma13051220.
The tendencies of development within the field of engineering materials show a persistent trend towards the increase of strength and toughness. This pressure is particularly pronounced in the field of steels, since they compete with light alloys and composite materials in many applications. The improvement of steels' mechanical properties is sought to be achieved with the formation of exceptionally fine microstructures ranging well into the nanoscale, which enable a substantial increase in strength without being detrimental to toughness. The preferred route by which such a structure can be produced is not by applying the external plastic deformation, but by controlling the phase transformation from austenite into ferrite at low temperatures. The formation of bainite in steels at temperatures lower than about 200 °C enables the obtainment of the bulk nanostructured materials purely by heat treatment. This offers the advantages of high productivity, as well as few constraints in regard to the shape and size of the workpiece when compared with other methods for the production of nanostructured metals. The development of novel bainitic steels was based on high Si or high Al alloys. These groups of steels distinguish a very fine microstructure, comprised predominantly of bainitic ferrite plates, and a small fraction of retained austenite, as well as carbides. The very fine structure, within which the thickness of individual bainitic ferrite plates can be as thin as 5 nm, is obtained purely by quenching and natural ageing, without the use of isothermal transformation, which is characteristic for most bainitic steels. By virtue of their fine structure and low retained austenite content, this group of steels can develop a very high hardness of up to 65 HRC, while retaining a considerable level of impact toughness. The mechanical properties were evaluated by hardness measurements, impact testing of notched and unnotched specimens, as well as compression and tensile tests. Additionally, the steels' microstructures were characterised using light microscopy, field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The obtained results confirmed that the strong refinement of the microstructural elements in the steels results in a combination of extremely high strength and very good toughness.
工程材料领域的发展趋势呈现出强度和韧性持续增加的态势。这种压力在钢铁领域尤为明显，因为在许多应用中，钢铁要与轻合金和复合材料竞争。人们试图通过形成范围深入到纳米级的异常精细的微观结构来提高钢铁的力学性能，这种微观结构能够在不损害韧性的情况下大幅提高强度。产生这种结构的首选途径不是通过施加外部塑性变形，而是通过控制在低温下从奥氏体到铁素体的相变。在低于约200°C的温度下在钢中形成贝氏体能够仅通过热处理获得块状纳米结构材料。与其他生产纳米结构金属的方法相比，这具有高生产率的优点，并且在工件的形状和尺寸方面限制较少。新型贝氏体钢的开发基于高硅或高铝合金。这些钢种具有非常精细的微观结构，主要由贝氏体铁素体板以及少量残余奥氏体和碳化物组成。这种非常精细的结构，其中单个贝氏体铁素体板的厚度可薄至5纳米，仅通过淬火和自然时效获得，无需使用大多数贝氏体钢所特有的等温转变。由于其精细的结构和低残余奥氏体含量，这类钢能够达到高达65 HRC的非常高的硬度，同时保持相当水平的冲击韧性。通过硬度测量、缺口和无缺口试样的冲击试验以及压缩和拉伸试验对力学性能进行了评估。此外，使用光学显微镜、场发射扫描电子显微镜（FESEM）和高分辨率透射电子显微镜（HRTEM）对钢的微观结构进行了表征。所得结果证实，钢中微观结构元素的强烈细化导致了极高强度和非常好的韧性的结合。
