Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts.

Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60 degrees to -60 degrees C, the yield strength was greater, leading to the enhancement of the toughness.

The change in the hydrogen-trapping behavior of a TiC particle accompanying its coherent to incoherent interfacial-character transition in a 0.05C-0.20Ti-2.0Ni steel that was quenched and tempered in a partially protective argon atmosphere and in ultrahigh vacuum (UHV) has been studied by thermal desorption spectrometry (TDS). The results indicated that (semi)coherent TiC precipitates demonstrate distinctly different hydrogen-trapping features from that of incoherent TiC particles with respect to hydrogen capacity, interaction energy with hydrogen, locations available for hydrogen occupation, and the capability of hydrogen absorption from the environment. The broad (semi)coherent interface of the disc-shaped (semi)coherent TiC precipitate does not trap hydrogen during tempering in a partially protected argon atmosphere, but traps hydrogen during cathodic charging at room temperature. The semicoherent interface traps 1.3 atoms/nm 2 of hydrogen at the core of the misfit dislocation with short-time charging (1 hour), which is characterized by a desorption activation energy of 55.8 kJ/mol. The side interface of the (semi)coherent TiC precipitate acts like the broad interface when the precipitate is small. As the precipitate grows, the side interface gradually loses its coherency and results in a simultaneous increase in the trapping activation energy and the binding energy. An increase in the trapping activation energy, i.e., the energy barrier for trapping, makes hydrogen trapping more difficult in cathodic charging at room temperature, while an increase in the binding energy enhances the capability of hydrogen absorption from the atmosphere during heat treatment. An incoherent TiC particle is not able to trap hydrogen during cathodic charging at room temperature due to its high energy barrier for trapping, but absorbs hydrogen during heat treatment at high temperatures. The amount of hydrogen that is trapped by incoherent TiC particles depends on their volume, which strongly indicates that incoherent TiC particles trap hydrogen within them rather than at the particle/matrix interface. Octahedral carbon vacancies are supposedly the hydrogen trap sites in incoherent TiC particles.

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Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts.

Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500°C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong a-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 lm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100°C to À40°C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

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