Nano-Bainitic Low Density Steel

对于当今的建筑来说，重量轻的高级钢材是必不可少的。铝作为钢中的合金元素降低了其密度，并已被用于生产低密度钢(LDS)，提供了高机械性能和低密度的有趣组合。此外，在保持钢的延展性的同时增加钢的强度，可以降低零件的尺寸，从而减轻它们的重量。这一战略已通过不同级别的先进高强度钢(AHSS)的开发来实施。高强度钢的组织可以是铁素体基钢、奥氏体钢或两者的混合组织。此外，高Al加入量导致了κ碳化物的存在，有时还会出现一些脆性有序相。在更高的温度下避免这些相的较大析出物是至关重要的，因为它们会对延展性产生负面影响。另一方面，据报道，在低温下产生这些相的细小弥散的析出物可以提高合金的强度。这种热处理循环类似于生产纳米贝氏体钢(Nb)的热处理循环，具有贝氏体铁素体和残余奥氏体片状纳米片层交替的组织以及块状奥氏体晶，提供了令人印象深刻的强度和塑性组合。然而，低相变温度需要较长的相变时间。该项目旨在开发LDS和Nb在合金化和热处理方面的协同作用，引入纳米贝氏体低密度钢的新概念，以及研究这一概念的界限和局限性，以便提供通过这些新引入的合金可实现的性能范围的全面概述。Nb和LDS之间的协同基于以下内容：1-高Al含量将显著加速贝氏体相变并增加形成的贝氏体铁素体的比例，从而在更快的相变时间获得更高的力学性能。2-将合金淬火到较低的等温温度将抑制在高温下形成大的不需要的脆性析出物，并且如果形成低温析出物，它们将是弥散分布的细小析出物，进一步强化组织。预计最终产品是具有贝氏体铁素体和残余奥氏体组织的高机械性能钢，密度低，并通过低成本的低温等温处理生产。研究了不同C(0.5~1.0wt.%)、Al(5.0~8.0wt.%)和Mn(1~8wt.%)对11种合金的显微组织、贝氏体相变动力学和力学性能的影响。这些结果与热力学模拟相结合，将被用于设计三种最终合金以及合适的加工路线，以实现最佳的最终性能。

Advanced grades of steel with reduced weight are essential for nowadays constructions. Aluminum as alloying element in steel reduces its density and has been used to produce low-density steels (LDS) offering an interesting combination of high mechanical properties at reduced densities. Moreover, increasing the strength of steels while maintaining their ductility enables down-gauging the parts, consequently lowering their weight. This strategy has been implemented via the development of advanced high strength steels (AHSS) in different levels.LDS have microstructures that could be ferrite based, austenite based, or a mixture of the two. Additionally, the high Al additions result in the presence of κ-carbides and – on occasion – a number of brittle ordered phases. It is essential to avoid larger precipitates of these phases at higher temperatures, as they negatively influence the ductility. On the other hand, generating fine well dispersed precipitates of these phases at low temperatures is reported to enhance the alloy’s strength. This heat treatment cycle is similar to that used to produce nano bainitic steels (nB) with microstructures of alternating nano-sized lamellae of bainitic ferrite and retained austenite, as well as blocky austenite grains, giving impressive strength and ductility combinations. However, the low transformation temperatures used necessitate long transformation times.The project aims to exploit the synergy between LDS and nB in terms of both alloying and heat treatment, introducing a novel concept of Nano-Bainitic Low Density Steel, as well as investigating the boundaries and limitations of this concept in order to provide an encompassing overview of the range of properties achievable via these newly introduced alloys.The synergy between nB and LDS is based on the following:1- High Al content will significantly accelerate the bainitic transformation and increase the fraction of bainitic ferrite formed leading to higher mechanical properties at faster transformation times.2- Quenching the alloys into low austempering temperatures will suppress the formation of large unwanted brittle precipitates at high temperatures, and if low temperature precipitates form, they will be well dispersed fine precipitates that further strengthen the microstructure.The foreseen final product is a high mechanical property steel with a microstructure of bainitic ferrite and retained austenite, having low density and produced by a low cost isothermal treatment at low temperature. The effect of varying C (between 0.5 – 1.0 wt.%), Al (5.0 – 8.0 wt.%) and Mn (1 – 8 wt.%) on the microstructure and bainitic transformation kinetics as well as the mechanical properties will be studied with eleven alloy combinations. These results, in combination with thermodynamic simulation will be used to design three final alloys along with suitable processing routes for optimal final properties.