Chemistry, design, and processing of two-stage TRIP steel

<p>"A regular solution model was developed to calculate the chemical driving force for α-martensite formation, ΔG^{λ→ α}_Chem. A model for the strain energy, ΔG^λ→α_str, was formulated utilizing the Young’s modulus (E), lattice misfit squared (δ²), and molar volume (Ω) which opposed the chemical driving force for α-martensite formation. The M^α_S was determined at a temperature at which ΔG^λ→α_Chem + ΔG^λ→α_str = 0. In conjunction with a previously developed ε-martensite model, a means of predicting the volume fraction of λ-austenite was determined; and it was shown that for values of ΔMs < 0, defined as Ms^ε - Ms^α produced the greatest amounts of retained γ-austenite in the as quenched microstructure. These models were tested, and confirmed, with a new alloy formulated to produce a steel with chromium replacing the traditional aluminum to obtain a ΔMs = -100 C° that exhibited the two-stage TRIP behavior. From this substitution the dynamic strain aging response could be mitigated through M₂₃(C,N)₆ precipitation trapping carbon and nitrogen. The work hardening behavior of these steels was found to be due to the Stage II (ε→α) martensitic reaction and not the dynamic strain aging of the steels. Eight medium-Mn steels were processed and it was found that when the intrinsic stacking fault energy was less than 10.5 mJ/m² the two-stage TRIP response was activated. Empirical relationships for the strength and ductility were determined for the two-stage TRIP steels. The developed models have been used to optimize alloy composition and a designed steel with composition Fe-13.8Mn-1.0Si-3.0Cr-0.15C-0.003N (wt. pct.) is recommended for future investigation"--Abstract, page iv.</p>

Name thesis:degree_name

Ph. D. in Metallurgical Engineering

Grantor

Missouri University of Science and Technology

Author dc:creator

• Field, Daniel M.

• EBSD
• Martensite
• Phase Transformation
• TRIP Steels
• Metallurgy

Repository record dc:identifier

https://scholarsmine.mst.edu/doctoral_dissertations/2672

OAI identifier oai:identifier

oai:scholarsmine.mst.edu:doctoral_dissertations-3677