Processing, microstructure and mechanical behavior of medium manganese steels
Bibliographic record
Abstract
Developing high strength steels has received increasing attention in steel industry, in order to meet the body-in-white weight reduction strategies pursued by the automotive industry for fuel economy and vehicle safety. In this context, medium manganese (Mn) steels containing 3 to 12 wt.% Mn are one of the strong candidates for the so-called third-generation advanced high-strength steels (AHSS), due to the superior mechanical properties exceeding the first-generation AHSS and the potentially lower cost compared to the high Mn second-generation AHSS. The ultrafine grain size and high fraction of retained austenite, adjustable to various TRIP (transformation-induced plasticity) and/or TWIP (twinning-induced plasticity) effects, constitute the key advantages of this group of steels. This thesis aims to develop a better understanding of the processing-microstructure-property relationship of medium Mn steels with Al and Si additions. Four experimental steels with compositions of Fe-0.2C-(7~10)Mn-3Al-(0~3)Si (in wt.%) were designed and examined. The phase transformation behavior, with respect to austenite formation and decomposition, of two hot rolled samples with 3 wt.% Al and 3 wt.% Si during intercritical annealing was firstly investigated to develop a better understanding of microstructural control for such steels. The microstructure of the leaner alloy with 7 wt.% Mn was substantially influenced by the annealing temperature, whereas the richer variant 10 wt.% Mn steel exhibited a stable ferrite-austenite duplex microstructure containing a fixed amount of retained austenite, which was found to be independent of the variations of annealing temperature. From these results, controlled annealing was performed for these two hot rolled samples, followed by a further processing by subjecting them to cold rolling and examining the microstructural evolution during cold rolling. Strain partitioning, between austenite and ferrite, and deformation twinning of austenite were found to critically influence the cold rollability of the two steels. Finally, the deformation and fracture mechanisms of the cold rolled and intercritical annealed medium Mn steels were studied, with particular emphasis on the effect of different Si additions (0~3 wt.%). Results showed that the mechanical property of the steels critically depended on the austenite fraction, TRIP and TWIP effect activated in austenite, as well as the strain partitioning between austenite and ferrite during deformation. The fracture mechanisms altered with different Si levels, which can change from dimpled-type fracture driven by void formation, mainly at the ferrite/strain-induced α′-martensite interfaces, to a combined dimple and cleavage/quasi- cleavage fracture, which was related to the brittle δ-ferrite in high Si alloys. The work also revealed a strong correlation between strain-induced martensite formation and the serrated plastic flow phenomenon associated with the Portevin-Le Chatelier effect (PLC) effect. Discontinuous behavior of strain-induced transformation was observed, which was due to the localized martensite formation within the PLC bands nucleating intermittently and propagating continuously during tensile straining.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".