Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels
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NTRODUCTION
ADVANCED high strength steels (AHSS) are of rapidly increasing commercial interest,[1] due to increasing global automobile production and heightened requirements for improved passenger safety and fuel efficiency.[2] Third-generation AHSS are attractive, particularly due to their enhanced mechanical properties relative to first-generation AHSS and lower cost than second-generation AHSS.[3] Due to their enhanced ductility and strength, third-generation AHSS can be used in thinner gauges, reducing body-in-white weight and consequently improving fuel efficiency. Unlike conventional automotive sheet steels, AHSS achieve improved ductility by suppressing strain localization with transformation-induced plasticity (TRIP).[4–7] In practice, a well-documented increase in tensile elongation occurs because of deformation-induced transformation from metastable austenite to martensite.[8–10] CHRISTOPHER B. FINFROCK, AMY J. CLARKE, and KESTER D. CLARKE are with the Colorado School of Mines, Advanced Steel Processing and Products Research Center, 1500 Illinois Street, Golden, CO 80401. Contact e-mail: cfi[email protected]. GRANT A. THOMAS is with the AK Steel Corporation, Steel Research & Innovation Center, 9227 Centre Pointe Drive, West Chester, OH 45069. Manuscript submitted October 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Quenching and partitioning (Q&P) processing, initially proposed by Speer et al.,[11–14] has been used to create steel microstructures that take advantage of TRIP by containing small fractions of retained austenite that are thermally stable at room temperature. In addition, these microstructures contain large-volume fractions of martensite, such that the yield strength is comparable to martensitic steels. Functionally, the yield strength is controlled largely by the presence of martensite, while work hardening and ductility are enhanced by TRIP. Many Q&P compositions contain carbon[15] for austenite stabilization, manganese[16,17] for austenite stabilization and solid solution strengthening, and silicon[18] to suppress the formation of carbides, which may otherwise reduce austenite stability by depleting carbon in austenite. By modifying either composition or Q&P heat treatment parameters, the microstructure and mechanical stability of retained austenite can be tailored, with the intention of optimizing deformation behavior. The role of austenite transformation on strain hardening rate is of particular interest for suppression of strain localization.[19, 20] Strain hardening has been characterized for individual phases in Q&P steels, with the assumption that Q&P steels behave as composite materials.[21–23] In Q&P processed material, strain hardening behavior has been explored in martensite[24] and
retained austenite.[25] Similar work has been performed for other AHSS, including manganese-enriched TRIP grades.[26] Deformation processing factors,[4,9,10,27–29] such as strain rate, strain state, and temperature, as well as microstructure factors,[30] such as distribution of carbon,[31] presence o
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