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Setting Mechanical Properties of High Strength Steels for Rapid Hot Forming Processes.

ABSTRACT: Hot stamping of sheet metal is an established method for the manufacturing of light weight products with tailored properties. However, the generally-applied continuous roller furnace manifests two crucial disadvantages: the overall process time is long and a local setting of mechanical properties is only feasible through special cooling techniques. Hot forming with rapid heating directly before shaping is a new approach, which not only reduces the thermal intervention in the zones of critical formability and requested properties, but also allows the processing of an advantageous microstructure characterized by less grain growth, additional fractions (e.g., retained austenite), and undissolved carbides. Since the austenitization and homogenization process is strongly dependent on the microstructure constitution, the general applicability for the process relevant parameters is unknown. Thus, different austenitization parameters are analyzed for the conventional high strength steels 22MnB5, Docol 1400M, and DP1000 in respect of the mechanical properties. In order to characterize the resulting microstructure, the light optical and scanning electron microscopy, micro and macro hardness measurements, and the X-ray diffraction are conducted subsequent to tensile tests. The investigation proves not only the feasibility to adjust the strength and ductility flexibly, unique microstructures are also observed and the governing mechanisms are clarified.

SUBMITTER: Lobbe C

PROVIDER: S-EPMC5502802 | biostudies-other | 2016 Mar

REPOSITORIES: biostudies-other

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Project description:High Al Low-density steels could have a transformative effect on the light-weighting of steel structures for transportation. They can achieve the desired properties with the minimum amount of Ni, and thus are of great interest from an economic perspective. In this study, the mechanical properties of two duplex low-density steels, Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8?C (wt.%) were investigated through nano-indentation and simulation through utilization of ab-initio formalisms in Density Functional Theory (DFT) in order to establish the hardness resulting from two critical structural features (?-carbides and B2 intermetallic) as a function of annealing temperature (500-1050?°C) and the addition of Ni. In the Ni-free sample, the calculated elastic properties of ?-carbides were compared with those of the B2 intermetallic Fe3Al-L12 and the role of Mn in the ? structure and its elastic properties were studied. The Ni-containing samples were found to have a higher hardness due to the B2 phase composition being NiAl rather than FeAl, with Ni-Al bonds reported to be stronger than the Fe-Al bonds. In both samples, at temperatures of 900?°C and above, the ferrite phase contained nano-sized discs of B2 phase, wherein the Ni-containing samples exhibited higher hardness, attributed again to the stronger Ni-Al bonds in the B2 phase. At 700?°C and below, the nano-sized B2 discs were replaced by micrometre sized needles of ? in the Ni-free sample resulting in a lowering of the hardness. In the Ni-containing sample, the entire ? phase was replaced by B2 stringers, which had a lower hardness than the Ni-Al nano-discs due to a lower Ni content in B2 stringer bands formed at 700?°C and below. In addition, the hardness of needle-like ?-carbides formed in ? phase was found to be a function of Mn content. Although it was impossible to measure the hardness of cuboid ? particles in ? phase because of their nano-size, the hardness value of composite phases, e.g. ??+?? was measured and reported. All the hardness values were compared and rationalized by bonding energy between different atoms.

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Setting Mechanical Properties of High Strength Steels for Rapid Hot Forming Processes.

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