S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Material Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
A Study on the Microstructure and Mechanical Behavior of High-Mn Low-density Steels
고망간 경량철강 소재의 미세조직과 기계적 거동에 대한 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Metals and alloys; Low-density steel; Aging; Beta-manganese; Vickers hardness; XRD; EPMA; EBSD; TEM; Morphological characteristics; Uniaxial tensile test; Fracture; Orientation relationship; Orientation relationship stereogram (OR stereogram); Nano-indentation; Hall-Petch relationship; Austenite
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2017. 2. 한흥남.
- In recent years, decreasing greenhouse gas emissions and improving fuel efficiency have been growing issues in the automotive and defense sectors. To achieve this goal, the development of advanced high-strength steels (AHSSs), such as dual-phase (DP) and transformation-induced plasticity (TRIP) steels has been long pursued for automotive applications. However, as the strength of steel increases, it gets more difficult to reduce the thickness due to the deterioration of structural stiffness, causing poor crashworthiness. In that sense, alternative concept of designing AHSS has been proposed mainly by lowering the density of steel while maintaining its high specific strength. Based on this concept, “low-density steels” have been developed containing light substitutional elements such as aluminum and/or silicon, which provide a density reduction effect by means of substitution and lattice expansion.
Low-density steels can be categorized into ferrite-based and austenite-based (including multiphase-based) alloys, depending on their constituent phases. Among them, the austenite-based low-density steels show superior mechanical properties and weight reduction rates. After these steels are aged at adequate temperature and time periods, many kinds of micro/nano-scale precipitates or intermetallic compounds are formed, such as κ-carbide, ordered bcc phases, and β-Mn. These precipitates have a profound effect on the mechanical properties of the low-density steels. However, there is still a paucity of characterization research on the β-Mn phase in terms of the formation behavior, microstructural evolution, and its effects on mechanical and fracture properties of austenite-based high-Mn low-density steels. Therefore, this research investigates the microstructure and mechanical behavior of high-Mn low-density steels after processing treatments, by using complementary microstructural and mechanical characterization.
Firstly, the microstructure and the intrinsic mechanical behavior of an austenite-based low-density steel containing 11.4 wt.% Al under aging heat treatment were investigated with respect to β-Mn precipitation. Lattice expansion of austenite and κ-carbide after β-Mn precipitation was observed by X-ray diffractometry (XRD) peak shift and electron probe micro-analyzer (EPMA). The Vickers hardness increased dramatically after an aging time of 1,000 minutes. The singular aging behavior also proceeded from precipitation of the β-Mn phase. The intrinsic mechanical property of austenite phase, which was obtained by nanoindentation, was correlated with the increase and inhomogeneous distribution of carbon in the austenite matrix.
Secondly, the β-Mn formation behavior of high-Mn low-density steels was investigated in terms of the morphological characteristics and alloying element distribution after aging treatments. A dramatic difference in the formation kinetics and morphology of β-Mn was observed depending on the addition of Al, which may increase the driving force for β-Mn formation. In addition, the effects of the aging process on the fracture behavior were examined in uniaxial tensile tests combined with microstructural observations. A severe loss of ductility resulted from the β-Mn formation and ordering of ferrite into the D03 phase, which was transformed before the β-Mn formation process.
Thirdly, the aging behavior and orientation relationships among constitutional phases and precipitates in Fe–31.4Mn–11.4Al–0.89C low-density steel were studied. The misorientation-angle distribution, Rodrigues–Frank vector space, and orientation relationship stereogram (OR stereogram) are used to elucidate the orientation relationships across γ-matrix/β-Mn and β-Mn/α-precipitate interphase boundaries. The orientation relationships obtained from the OR stereograms were clarified by checking the deviation angle distributions of interface segments from the ideal orientation relationships, as well as the TEM diffraction patterns at the interface boundaries. In addition, from both orientation relationships for γ-matrix/β-Mn and β-Mn/α-precipitate interfaces, the interface character between γ-matrix and α-precipitate is examined and compared to conventional fcc/bcc orientation relationships.
Lastly, the deformation behavior of duplex low-density steel was analyzed by correlation between macro-scale uniaxial tension and nano-scale indentation. A dramatic difference in the tensile behavior was observed between two specimens obtained under specific heat-treatment conditions, despite the same chemical composition and similar microstructures. In order to understand this difference, the intrinsic mechanical properties of each phase were analyzed based on nano-indentation results considering Hall-Petch relationship. In addition, the mechanical stability of retained austenite was investigated by in-situ electron backscattered diffraction.
From this study, the β-Mn phase in austenite-based high-Mn low-density steels was described well, in terms of the formation behavior, microstructural evolution, orientation relationships, and its effects on mechanical and fracture properties. In addition, the dramatic differences of the macroscopic tensile behavior regarding the yield strength, yield point phenomenon, and strain hardening in the two duplex low-density steels was successfully explained.