Cost effective low temperature bainite

为了满足日益增长的对高强度和高韧性的先进材料的需求，在汽车、轴承、齿轮和铁路等许多应用领域，具有良好的高强韧性组合的无碳化物贝氏体钢已经被广泛生产出来。通过在钢中加入约1.5wt%的硅来抑制贝氏体形成过程中渗碳体的析出，可以获得优异的性能。为了发展这种组织，人们在低温下形成贝氏体，即现在所说的低温贝氏体(LTB)。这种类型的贝氏体由铁素体和奥氏体片状的纳米级相间组成。这种钢的超高强度是其细小的贝氏体组织的结果。观察到的细化主要是由于低温转变温度能够提高奥氏体的强度；后者导致形成的贝氏体板细化。从商业角度来看，两个相关因素限制了LTB钢的推广，尽管它们具有优异的力学性能，即低温贝氏体转变速度缓慢和使用昂贵的合金化元素。在整个项目中，计划引入新的合金化和热机械加工概念，通过减少/消除高成本的合金化元素，如钴、铬和镍，并重新设计合金和加速贝氏体相变速率的工艺来降低LTB钢的成本。建议的锰和碳含量是为了：-降低钢的马氏体开始温度，以便贝氏体相变可以在较低的温度下进行。-确保合金在退火过程中获得完全奥氏体的适宜性，即所采用的高铝水平限制了奥氏体的形成，并使图表上的奥氏体区收缩到称为伽马环的小区域。还计划通过以下处理来改善低碳钢的机械性能：-利用低碳钢的BH-潜力来提高其屈服强度。对这种材料进行的预试验表明，BH2值约为250兆帕，在一些调查的情况下，屈服强度超过2 GPA。这种非常高的屈服强度将为设计工程师打开新的视野。计划开发一个基于物理的模型，用于预测时效对LTB钢最终性能的影响。-通过应用不同的热机械加工(TMP)路线在平板产品中生产LTB。在这方面，通过应用设计的热变形计划来生产含有嵌入在LTB的奥氏体岛中的形变诱导的细小铁素体晶的组织。变形膨胀仪将被用来定义在进入大型拉伸和冲击样品的生产之前所需的热机械制度以产生所需的微观组织。

To satisfy the ever-increasing need for advanced materials fulfilling the demands for both, high strength and high toughness, in many application fields such as automotive, bearing, gear and railways, carbide-free bainite steels with excellent combination of high strength and toughness, have been widely manufactured. The excellent properties are achieved by alloying the steel with about 1.5 wt% Si to suppress the precipitation of cementite during bainite formation.Many research efforts are exerted to develop this structure by forming the bainite at low temperatures, currently known as low temperature bainite (LTB). This type of bainite is composed of alternating, nano-sized plates of ferrite and austenite. The ultra-high strength of this steel is a result of its fine bainite structure. The observed refinement is a consequence mainly of the ability of low transformation temperature to enhance the strength of austenite; the latter effect causes refinement of the formed bainite plates.From the commercial point of view, two dependent factors limit the spread of LTB steels in spite of their excellent mechanical properties namely, the slow rate of bainitic transformation at low temperature and the use of expensive alloying elements. Throughout this project, it is planned to introduce new alloying and thermo-mechanical processing concepts to decrease the cost of the LTB steel by reducing/eliminating high-cost alloying elements, e.g. Co, Cr and Ni and redesigning the alloy and the process to accelerate the bainite transformation rate.In this context a compromise among Al, Mn and C content is proposed to:- Suppress the martensite start temperature of the steel so that the bainite transformation can be performed at lower temperatures.- Assure the amenability of the alloy to obtain fully austenite during annealing that is the adopted high Al-level restricts the formation of austenite and causes the austenite area of the diagram to contract to a small area referred to as the gamma loop.It is also planned to improve mechanical properties of LTB through processing by:- Exploiting the BH-potential of the LTB to improve its yield strength. Pre-tests carried out on such material showed a BH2 value of about 250 MPa which shifted the yield strength in some investigated cases to exceed 2 GPa. This very high yield strength would open new horizons for design engineers. A physical-based model for predicting the influence of aging on final properties of the LTB steels is planned to be developed.- Producing the LTB in flat products by applying different thermo-mechanical processing (TMP) routes. In this respect, a structure containing deformation induced fine ferrite grains embedded in the austenite islands of LTB is to by produced applying designed TMP.The deformation dilatometer will be used to define the thermal-mechanical schedule required to produce the desired microstructures before moving to the production of large tensile and impact samples.