Nano-Bainitic Low Density Steel

减轻重量的高级钢材对现在的建筑是必不可少的。铝作为钢中的合金元素降低了其密度，并被用于生产低密度钢（LDS），在低密度下提供了高机械性能的有趣组合。此外，在保持延展性的同时提高钢的强度，可以减小零件的尺寸，从而降低它们的重量。这一战略已通过开发不同级别的先进高强度钢（AHSS）来实施。LDS的微观结构可以是铁素体基、奥氏体基或两者的混合。此外，高Al的加入导致了κ-碳化物的存在，有时还会产生一些脆性有序相。必须避免在较高温度下这些相的较大析出，因为它们对延展性有负面影响。另一方面，据报道，在低温下产生这些相的细小而分散的析出物可以提高合金的强度。这种热处理循环类似于用于生产纳米贝氏体钢（nB）的热处理循环，其显微结构是贝氏体铁素体和残余奥氏体的纳米片状交替，以及块状奥氏体晶粒，具有令人印象深刻的强度和延展性组合。然而，较低的转变温度需要较长的转变时间。该项目旨在利用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，并研究这一概念的界限和局限性，以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之间的协同作用基于以下方面：1-高Al含量会显著加速贝氏体转变，增加贝氏体铁素体的形成比例，从而在更快的转变时间内获得更高的力学性能。2-将合金淬火至低等温，可抑制高温下大量不需要的脆性析出物的形成，若形成低温析出物，则为分散良好的细小析出物，进一步强化组织。预期的最终产品是一种高机械性能钢，其显微组织为贝氏体铁素体和残余奥氏体，具有低密度，通过低成本的低温等温处理生产。研究了C （0.5 - 1.0 wt.%）、Al （5.0 - 8.0 wt.%）和Mn （1 - 8 wt.%）对合金组织和贝氏体转变动力学以及力学性能的影响。这些结果与热力学模拟相结合，将用于设计三种最终合金以及合适的加工路线，以获得最佳的最终性能。

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.