2021 Vol. 28, No. 5
Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles. Different from the popular cold stamping, hot forming of boron-alloyed manganese steels, such as 22MnB5, could produce ultra-high-strength steel parts without springback and with accurate control of dimensions. Moreover, hot-formed medium-Mn steels could have many advantages, including better mechanical properties and lower production cost, over hot-formed 22MnB5. This paper reviews the hot forming process in the automotive industry, hot-formed steel grades, and medium-Mn steel grades and their application in hot forming in depth. In particular, the adaptabilities of medium-Mn steels and the presently popular 22MnB5 into hot forming were compared thoroughly. Future research should focus on the technological issues encountered in hot forming of medium-Mn steels to promote their commercialization.
The high strength martensite steels are widely used in aerospace, ocean engineering, etc., due to their high strength, good ductility and acceptable corrosion resistance. This paper provides a review for the influence of microstructure on corrosion behavior of high strength martensite steels. Pitting is the most common corrosion type of high strength stainless steels, which always occurs at weak area of passive film such as inclusions, carbide/intermetallic interfaces. Meanwhile, the chromium carbide precipitations in the martensitic lath/prior austenite boundaries always result in intergranular corrosion. The precipitation, dislocation and grain/lath boundary are also used as crack nucleation and hydrogen traps, leading to hydrogen embrittlement and stress corrosion cracking for high strength martensite steels. Yet, the retained/reversed austenite has beneficial effects on the corrosion resistance and could reduce the sensitivity of stress corrosion cracking for high strength martensite steels. Finally, the corrosion mechanisms of additive manufacturing high strength steels and the ideas for designing new high strength martensite steel are explored.
Four types of meager and meager-lean coal and one type of high-quality anthracite were selected based on the safety requirements for blast furnace coal injection and domestic coal quality to conduct microstructure and component analyses. The analyses of the organic and inorganic macerals and the chemical compositions of the selected coal samples indicate that the four types of meager and meager-lean coal have low volatilization, low ash content, and low sulfur content; these qualities are suitable for blast furnace injection. Grindability test was conducted on the four types of meager and meager-lean coal and the anthracite mixed coal samples. Results indicate that the mixture of meager and meager-lean coal and anthracite is beneficial to improve the grindability of pulverized coal. The explosive tests reveal that the selected coal samples are non-explosive or weakly explosive. When the proportion of meager and meager-lean coal is less than 40wt%, the mixed coal powder would not explode during the blowing process. The minimum ignition temperature test determines that the minimum ignition temperatures of the four types of meager and meager-lean coal and anthracite are 326, 313, 310, 315, and 393°C, respectively. This study provides a guiding research idea for the safety of meager and meager-lean coal used in blast furnace injection.
In the Collaborative Research Centre 761’s “Steel ab initio - quantum mechanics guided design of new Fe based materials,” scientists and engineers from RWTH Aachen University and the Max Planck Institute for Iron Research conducted research on mechanism-controlled material development with a particular focus on high-manganese alloyed steels. From 2007 to 2019, a total of 55 partial projects and four transfer projects with industrial participation (some running until 2021) have studied material and process design as well as material characterization. The basic idea of the Collaborative Research Centre was to develop a new methodological approach to the design of structural materials. This paper focuses on selected results with respect to the mechanical properties of high-manganese steels, their underlying physical phenomena, and the specific characterization and modeling tools used for this new class of materials. These steels have microstructures that require characterization by the use of modern methods at the nm-scale. Along the process routes, the generation of segregations must be taken into account. Finally, the mechanical properties show a characteristic temperature dependence and peculiarities in their fracture behavior. The mechanical properties and especially bake hardening are affected by short-range ordering phenomena. The strain hardening can be adjusted in a never-before-possible range, which makes these steels attractive for demanding sheet-steel applications.
We investigated the effect of Al2O3 content on the viscosity of CaO–SiO2–Al2O3–8wt%MgO–1wt%Cr2O3 (mass ratio of CaO/SiO2 is 1.0, and Al2O3 content is 17wt%–29wt%) slags. The results show that the viscosity of the slag increases gradually with increases in the Al2O3 content in the range of 17wt% to 29wt% due to the role of Al2O3 as a network former in the polymerization of the aluminosilicate structure of the slag. With increases in the Al2O3 content from 17wt% to 29wt%, the apparent activation energy of the slags also increases from 180.85 to 210.23 kJ/mol, which is consistent with the variation in the critical temperature. The Fourier-transform infrared spectra indicate that the degree of polymerization of this slag is increased by the addition of Al2O3. The application of Iida’s model for predicting the slag viscosity in the presence of Cr2O3 indicates that the calculated viscosity values fit well with the measured values when both the temperature and Al2O3 content are at relatively low levels, i.e., the temperature range of 1673 to 1803 K and the Al2O3 content range of 17wt%–29wt% in CaO–SiO2–Al2O3–8wt%MgO–1wt%Cr2O3 slag.
The mechanism of oxide inclusions in fatigue crack initiation in the very-high cycle fatigue (VHCF) regime was clarified by subjecting bearing steels deoxidized by Al (Al-deoxidized steel) and Si (Si-deoxidized steel) to ultrasonic tension–compression fatigue tests (stress ratio, R = −1) and analyzing the characteristics of the detected inclusions. Results show that the main types of inclusions in Si- and Al-deoxidized steels are silicate and calcium aluminate, respectively. The content of calcium aluminate inclusions larger than 15 μm in Si-deoxidized steel is lower than that in Al-deoxidized steel, and the difference observed may be attributed to different inclusion generation processes during melting. Despite differences in their cleanliness and total oxygen contents, the Si- and Al-deoxidized steels show similar VHCF lives. The factors causing fatigue failure in these steels reveal distinct differences. Calcium aluminate inclusions are responsible for the cracks in Al-deoxidized steel. By comparison, most fatigue cracks in Si-deoxidized steel are triggered by the inhomogeneity of a steel matrix, which indicates that the damage mechanisms of the steel matrix can be a critical issue for this type of steel. A minor portion of the cracks in Si-deoxidized steel could be attributed to different types of inclusions. The mechanisms of fatigue fracture caused by calcium aluminate and silicate inclusions were further analyzed. Calcium aluminate inclusions first separate from the steel matrix and then trigger crack generation. Silicate inclusions and the steel matrix are closely combined in a fatigue process; thus, these inclusions have mild effects on the fatigue life of bearing steels. Si/Mn deoxidation is an effective method to produce high-quality bearing steel with a long fatigue life and good liquid steel fluidity.
Here we present a novel approach of intercritical heat treatment for microstructure tailoring, in which intercritical annealing is introduced between conventional quenching and tempering. This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite, resulting in a low yield ratio (YR) and high impact toughness in a high-strength low-alloy steel. The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing, in comparison to the steel with full martensitic microstructure. The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering. The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases, but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.
A numerical study of stress distribution and fatigue behavior in terms of the effect of voids adjacent to inclusions was conducted with finite element modeling simulations under different assumptions. Fatigue mechanisms were also analyzed accordingly. The results showed that the effects of inclusions on fatigue life will distinctly decrease if the mechanical properties are close to those of the steel matrix. For the inclusions, which are tightly bonded with the steel matrix, when the Young’s modulus is larger than that of the steel matrix, the stress will concentrate inside the inclusion; otherwise, the stress will concentrate in the steel matrix. If voids exist on the interface between inclusions and the steel matrix, their effects on the fatigue process differ with their positions relative to the inclusions. The void on one side of an inclusion perpendicular to the fatigue loading direction will aggravate the effect of inclusions on fatigue behavior and lead to a sharp stress concentration. The void on the top of inclusion along the fatigue loading direction will accelerate the debonding between the inclusion and steel matrix.
The influences of hydrogen on the mechanical properties and the fracture behaviour of Fe–22Mn–0.6C twinning induced plasticity steel have been investigated by slow strain rate tests and fractographic analysis. The steel showed high susceptibility to hydrogen embrittlement, which led to 62.9% and 74.2% reduction in engineering strain with 3.1 and 14.4 ppm diffusive hydrogen, respectively. The fracture surfaces revealed a transition from ductile to brittle dominated fracture modes with the rising hydrogen contents. The underlying deformation and fracture mechanisms were further exploited by examining the hydrogen effects on the dislocation substructure, stacking fault probability, and twinning behaviour in pre-strained slow strain rate test specimens and notched tensile specimens using coupled electron channelling contrast imaging and electron backscatter diffraction techniques. The results reveal that the addition of hydrogen promotes planar dislocation structures, earlier nucleation of stacking faults, and deformation twinning within those grains which have tensile axis orientations close to <111>//rolling direction and <112>//rolling direction. The developed twin lamellae result in strain localization and micro-voids at grain boundaries and eventually lead to grain boundary decohesion.
Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility. This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite. Such a unique microstructure is processed by intercritical annealing, where austenite reversion occurs in a fine martensitic matrix. In the present study, austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS® coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2. In particular, a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite. The simulated microstructural morphology and phase transformation kinetics (indicated by the amount of phase) concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction, respectively.
This study aims to discover the stress-state dependence of the dynamic strain aging (DSA) effect on the deformation and fracture behavior of high-strength dual-phase (DP) steel at different deformation temperatures (25–400°C) and reveal the damage mechanisms under these various configurations. To achieve different stress states, predesigned specimens with different geometric features were used. Scanning electron microscopy was applied to analyze the fracture modes (e.g., dimple or shear mode) and underlying damage mechanism of the investigated material. DSA is present in this DP steel, showing the Portevin–Le Chatelier (PLC) effect with serrated flow behavior, thermal hardening, and blue brittleness phenomena. Results show that the stress state contributes distinctly to the DSA effect in terms of the magnitude of thermal hardening and the pattern of blue brittleness. Either low stress triaxiality or Lode angle parameter promotes DSA-induced blue brittleness. Accordingly, the damage mechanisms also show dependence on the stress states in conjunction with the DSA effect.
Effects of the weld microstructure and inclusions on brittle fracture initiation are investigated in a thermally aged ferritic high-nickel weld of a reactor pressure vessel head from a decommissioned nuclear power plant. As-welded and reheated regions mainly consist of acicular and polygonal ferrite, respectively. Fractographic examination of Charpy V-notch impact toughness specimens reveals large inclusions (0.5–2.5 μm) at the brittle fracture primary initiation sites. High impact energies were measured for the specimens in which brittle fracture was initiated from a small inclusion or an inclusion away from the V-notch. The density, geometry, and chemical composition of the primary initiation inclusions were investigated. A brittle fracture crack initiates as a microcrack either within the multiphase oxide inclusions or from the debonded interfaces between the uncracked inclusions and weld metal matrix. Primary fracture sites can be determined in all the specimens tested in the lower part of the transition curve at and below the 41-J reference impact toughness energy but not above the mentioned value because of the changes in the fracture mechanism and resulting changes in the fracture appearance.
Nanoindentation is an attractive characterization technique, as it not only measures the local properties of a material but also facilitates understanding of deformation mechanisms at submicron scales. However, because of the complex stress–strain field and the small scale of the deformation under the nanoindenter, the results can be easily influenced by artifacts induced during sample preparation. In this work, a systematic study was conducted to better understand the influence of sample preparation methods on the nanoindentation results of ductile metals. All experiments were conducted on a steel (Fe–22Mn–0.65C, wt%) with twinning-induced plasticity (TWIP), which was selected for its large grain size and sensitivity to different surface preparation methods. By grouping the results obtained from each nanoindent, chemical polishing was found to be the best sample preparation method with respect to the resulting mechanical properties of the material. In contrast, the presence of a deformation layer left by mechanical polishing and surface damage induced by focused ion beam (FIB) scanning were confirmed by the dislocation-nucleation-induced pop-in events of nanoindentation.
(CoCrFeNi)95Nb5 high entropy alloy (HEA) coatings were successfully fabricated on a substrate of Q235 steel by laser cladding technology. These (CoCrFeNi)95Nb5 HEA coatings possess excellent properties, particularly corrosion resistance, which is clearly superior to that of some typical bulk HEA and common engineering alloys. In order to obtain appropriate laser cladding preparation process parameters, the effects of laser energy density on the microstructure, microhardness, and corrosion resistance of (CoCrFeNi)95Nb5 HEA coating were closely studied. Results showed that as the laser energy density increases, precipitation of the Laves phase in (CoCrFeNi)95Nb5 HEA coating gradually decreases, and diffusion of the Fe element in the substrate intensifies, affecting the integrity of the (CoCrFeNi)95Nb5 HEA. This decreases the microhardness of (CoCrFeNi)95Nb5 HEA coatings. Moreover, the relative content of Cr2O3, Cr(OH)3, and Nb2O5 in the surface passive film of the coating decreases with increasing energy density, causing corrosion resistance to decrease. This study demonstrates the controllability of a high-performance HEA coating using laser cladding technology, which has significance for the laser cladding preparation of other CoCrFeNi-system HEA coatings.