Cost effective low temperature bainite

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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.