Cost effective low temperature bainite

为了满足对同时满足高强度和高韧性要求的先进材料的日益增长的需求，在许多应用领域如汽车、轴承、齿轮和铁路中，具有高强度和高韧性的优异组合的无碳化物贝氏体钢已被广泛制造。通过在钢中加入约1.5wt%的Si来抑制贝氏体形成过程中的贝氏体析出，从而获得优异的性能。许多研究努力通过在低温下形成贝氏体来开发这种组织，目前称为低温贝氏体（LTB）。这种类型的贝氏体由铁素体和奥氏体的交替的纳米尺寸板组成。这种钢的超高强度是其精细贝氏体组织的结果。观察到的细化主要是一个后果的能力，低的转变温度，以提高奥氏体的强度;后者的效果导致细化形成的贝氏体pipels.From商业的角度来看，两个独立的因素限制了LTB钢的传播，尽管它们的优良的机械性能，即，在低温下的贝氏体转变的速度慢，使用昂贵的合金元素。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/消除高成本的合金元素，如Co、Cr和Ni，并重新设计合金和工艺来降低LTB钢的成本，以加速贝氏体转变速率。在此背景下，建议在Al、Mn和C含量之间进行折衷，以：- 抑制钢的马氏体起始温度，使得贝氏体转变可以在较低温度下进行。确保合金在退火过程中获得完全奥氏体的可接受性，即采用高Al水平限制奥氏体的形成，并导致图中的奥氏体区域收缩到称为伽马环的小区域。还计划通过以下处理来改善LTB的机械性能：-开发LTB的BH电位以提高其屈服强度。对这种材料进行的预测试显示出约250 MPa的BH 2值，其在某些研究情况下使屈服强度超过2 GPa。这种非常高的屈服强度将为设计工程师开辟新的视野。计划开发一种基于物理的模型，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）路线在扁平产品中生产LTB。在这方面，包含嵌入在LTB的奥氏体岛中的变形诱导的细铁素体晶粒的结构将通过应用设计的TMP来产生。在移动到大的拉伸和冲击样品的生产之前，变形温度计将用于定义产生所需的微观结构所需的热机械时间表。

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.