Date of Award
Doctor of Philosophy (PhD)
David R. Johnson
Committee Member 1
Matthew J. M. Krane
Committee Member 2
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). The higher strength of these steels allows for thinner gauge components to be implemented, reducing the weight of the vehicle and increasing the fuel economy. Included under the umbrella of AHSS are alloys containing small amounts of boron, 0.002 – 0.005 wt% B. The addition of boron leads to difficulties during commercial production, specifically via continuous casting.
The casting difficulties are predicted to stem from a metatectic reaction, → + 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. Both experimental methods show evidence of a metatectic reaction providing a good comparison to predicted phase diagrams. Based on the experimental results, the metatectic reaction occurs over a broader range of compositions than predicted, 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 -bcc to -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 directionally solidified zone remained below 5 ppm B, it has an impact on the microstructure as bainite forms where proeutectoid ferrite and pearlite are predicted from the carbon composition.
With commercial boron-containing AHSS alloys, levitation zone melting was used to investigate how boron segregation was affected by the presence of other solute elements. Although boron is known to be a strong nitride former, boron was found to precipitate out with iron and carbon while nitrogen formed precipitates with titanium and carbon. When low amounts of boron are present, less than 5 ppm B, it was seen to have an influence on the microstructure development, similar to the ternary alloy. However, higher levels of boron result in the formation of boride precipitates along original austenite grain boundaries as a result of boron-rich intercellular liquid, not from a solid state transformation. In commercial castings, the formation of the low melting boride phase from interdendritic segregation is likely a source of the defects.
Luitjohan, Kara, "Boron Segregation and its Effects in Advanced High Strength Steel" (2018). Open Access Dissertations. 2010.