Please consider attending the following: MATERIALS ENGINEERING “Boron Segregation and its Effects in Advanced High Strength Steels” By Kara E. Luitjohan Purdue MSE Ph.D. Final Exam Advisors: Professor David R. Johnson and Professor Volkan Ortalan ABSTRACT There is a current push in the automotive industry to increase the fuel economy of passenger vehicles. One method to achieve this goal is through the use of advanced high strength steels (AHSS). Included under the umbrella of AHSS are alloys containing small amounts of boron, 0.002 - 0.005 wt% B. This addition of boron leads to difficulties during commercial production, specifically via continuous casting. The casting difficulties are predicted to stem from a metatectic reaction, δ → γ + L, occurring in the iron-boron binary system. Depending on which thermodynamic database is utilized, this reaction is predicted to occur at different boron levels. To experimentally investigate the predicted metatectic reaction, levitation zone melting is used to control the boron segregation in two simple Fe-B binary alloys, and confocal scanning laser microscopy allows for in-situ observation of local microstructural changes as temperature is varied. Based on the experimental results, the metatectic reaction occurs over a broader range of compositions than predicted with the reaction occurring from 0.0025 to ∼0.06 wt% B. The experimental technique successfully applied to the binary system is then used to investigate how the addition of carbon and other solute elements affect the metatectic reaction and subsequent solidification. When carbon is added to the Fe-B system, levitation zone melting results in a δ-bcc to γ-fcc peritectic jump as steady-state growth conditions are reached with respect to carbon segregation. Boron remains in solution until the last zone to solidify. Although the boron content throughout the ingot remains below 5 ppm B, it has an impact on the microstructure as bainite forms where proeutectoid ferrite and pearlite are predicted base on the carbon composition. When other solute elements are added in three different boron-containing commercial steels, the controlled solidification conditions of levitation zone melting are not enough to prevent the formation of a low melting temperature boron-rich phase throughout the directionally solidified ingots. These ingots display a peritectic jump, similar to the ternary case, followed by a breakdown of the planar solidification front leading to the formation of boron-rich intercellular liquid. During dendritic solidification, as seen in continuous casting, boron-rich inter-dendritic liquid can be predicted to form which will result in a low melting boron-rich phase that could be the source of the casting defects. Date: Monday, July 16, 2018 Time: 2:00 P.M. Place: ARMS 1028