Static strain aging phenomena in cold-rolled dual-phase steels

The effect of low-temperature aging, with aging temperatures up to 170 °C, on a cold-rolled CMn-CrMo dual-phase (DP) ferrite-martensite steel was investigated. This material was processed using three different intercritical annealing treatments, leading to DP structures with different microstructures and properties. It has been found that both the aging in the ferrite phase and the tempering in the martensite play an important role in the mechanical behavior of the material with regard to the strain aging phenomena. The yield stress increase accompanying the aging phenomenon revealed three separate aging stages. In the present study, those stages were determined to be the result of the pinning of dislocations in the ferrite, the C-cluster formation, or low-temperature carbide precipitation in the ferrite and the volume contraction of the martensite due to formation of low-temperature carbides, leading to the relief of residual stresses in the ferrite. In the absence of a clear yield point, a new method is proposed to measure the increase in yield stress due to aging only.     

Strength,strain capacity and toughness of five dual-phase pipeline steels

The effect of microstructures on strength, strain capacity and low temperature toughness of a micro-alloyed pipeline steel was elucidated. Five various dual-pha...     

Kinetics of strain aging in drawn pearlitic steels

The kinetics of strain aging in a drawn pearlitic steel wire were analyzed based on measurements from three different experimental techniques: tensile testing, electrical resistivity, and internal friction. The progress of aging after a total reduction in area of 86 pct by drawing and heating between 60 °C and 200 °C was evaluated in terms of the changes in yield strength, electrical resistivity, and temperature-dependent background damping. The kinetic law and the apparent activation energies of the processes occurring in this temperature range were determined by the isothermal variation of the transformed fraction, obtained from the changes in properties with aging time. Under the conditions considered, static strain aging of drawn pearlitic steels seems to occur in two distinct stages, each associated with different atomic mechanisms. The first stage takes place between 60 °C and 100 °C and is characterized by a small increase in yield strength and a decrease in electrical resistivity and background damping. The second stage of aging occurs at higher temperatures or longer aging times and is marked by a sharp increase in yield strength and an increase in electrical resistivity, while the background damping reaches very small values. The probable mechanisms related to this behavior are discussed in terms of the empirical law and the apparent activation energy found.     

Static strain aging of α-Fe-P alloys

Interstitial-free Fe-P alloys (0.083, 0.23 and 0.40 at. pct P) and a rephosphorized low-carbon steel were strained 4 pct in tension, aged isothermally at temperatures between 373 and 723 K, then strained to fracture at room temperature. The yield strength increment of the Fe-P alloys increases with aging time, reaches a maximum, then decreases. As with other substitutional solutes, only the first stage of strain aging was seen; the UTS and elongation remained unchanged. Phosphorus is more effective than other substitutional solutes in causing strain aging. Because of the limited solubility of P in α-Fe, increasing creasing the P content from 0.083 to 0.40 at. pct had only a slight effect on strain aging. The activation energy for strain aging is about 220 kJ/mole, indicating that aging is controlled by lattice diffusion of P. When both interstitial and substitutional solutes are present, as in the rephosphorized steel, aging by interstitials masks any aging by P. E. J. DZIURA, formerly Graduate Student, Department of Materials and Metallurgical Engineering, University of Michigan, is now with the     

Hydrogen embrittlement of dual-phase steels

Reversible hydrogen embrittlement (HE) is usually only found in quenched and tempered steels with yield stresses in excess of 1035 MPa (150 ksi). A study of the HE phenomena in two dual-phase steels with tensile strengths of about 690 MPa (100 ksi) has shown that these steels are susceptible to the presence of hydrogen. HE results in a reduction in fracture strength, although no preyield failures are observed, and a change in fracture mode from ductile dimpling to transgranular cleavage. After prestraining and HE, it is found that the greater the prestrain the higher is the fracture stress. It is concluded that the presence of the 15 to 20 pct high carbon (0.6 pct C) high strength martensite in the dual-phase steels is responsible for the HE; tempering studies give results consistent with this idea. Delayed failure tests on notched specimens showed that for the as-received condition, the run-out stress (stress for no failures in 50 to 100 h) to be above the macroscopic flow stress. A condition for HE failure in dual-phase steels appears to be considerable macroscopic deformation.     

Deformation characteristics of dual-phase steels

An investigation was made to determine the effect of volume fraction of martensite on the strength, ductility, and fracture behavior of dual-phase steels. For both V-and Mo-bearing alloys, a linear relationship was found to exist between the amount of martensite and both tensile strength and uniform elongation. Contrary to previous reports that predict microcracking of martensite above 0.2 volume fraction to be responsible for a drop-off in ductility, no martensite cracking was found. Fracture was caused by void formation at small inclusions or at the ferrite/martensite interface. In severely banded structures, the strength/ductility relationship was detrimentally affected because of the tendency for crack propagation in the less ductile martensite without the cracks being blunted by the ferrite matrix.     

Early stages of yielding and strain aging of a vanadium-containing dual-phase steel

A study has been made of the early stages of yielding, 10⁻⁵ to 10⁻² plastic strain range, of a vanadium containing dual-phase steel as a function of pct martensite, pct prestrain and aging time and temperature. It is found that the 10⁻⁵ yield stress is independent of the pct martensite, while the 10⁻³ flow stress is proportional to, and increases rapidly with the martensite content. For dual-phase steels of commercial interest (15 to 25 pct martensite content), it is concluded that the yield behavior is controlled by the free dislocations introduced into the ferrite matrix when the austenite regions transform to martensite and their subsequent interaction with the martensite regions. While straining increases the microyield stresses and aging increases both the micro and macroyield stresses, these processes are not additive in a strained plus aged specimen. The activation energy for the production of a yield point plateau was determined to be about 33 kcal/mol. It is proposed that this high value is a result of carbon atoms interacting with vanadium rich clusters which remain in the structure after vanadium carbide precipitates are dissolved during intercritical annealing.     

Texture development in dual-phase cold-rolled 18 pct Ni maraging steel

Austenite and martensite textures were studied in 18 pct Ni 350-maraging steel as a function of various degrees of cold rolling. The austenite phase in the samples was produced by repeated thermal cycling between ambient and 800 °C. The austenite phase thus formed was mechanically unstable and transformed to the martensite phase after 30 pct cold rolling. The texture developed as a result of cold rolling, and its effect upon microstructure and hardness has been studied.     

Graphite formation in high-purity cold-rolled carbon steels

Low-carbon (0.06 pet C) and high-carbon (0.50 pet C) Al-killed (AK) steels were prepared by melting high-purity electrolytic iron. They were hot-rolled, cold-rolled, and then batch-annealed for the investigation of microstructure and mechanical properties. It was found that graphitization of cementite took place in steels of extremely low phosphorus and sulfur contents. Graphite was thought to originate in voids formed by cracking of cementite during the cold rolling. It was speculated that in steels of commercial purity, phosphorus and sulfur segregated to the void surface and, thus, suppressed graphitization during the annealing. It was also found that the high-carbon steels became as soft and ductile as low-carbon steels by graphitization.     

On minimizing the bauschinger effect in steels by dynamic strain aging

The Bauschinger effect was measured in the conventional tension-compression mode in five steels, AISI 1020, 1522, and 1035, ASTM A374 (Cor-Ten) and an HSLA Mn-Mo-Cb steel. The results were compared with those obtained by performing the forward strain at a temperature near the peak of dynamic strain aging for each of the steels and the subsequent reverse strain at room temperature. The dynamic strain aging process sharply reduced the extent of the Bauschinger effect and was particularly effective in re-storing the elastic portion of the flow curve during the reverse strain. We conclude that the straightening of steel parts after heat treatment should be done warm, in the dynamic strain aging range, whenever feasible. Formerly students at the University of Michigan.     

Hot rolling texture development in CMnCrSi dual-phase steels

The amount of strain below the temperature of nonrecrystallization, T _nr , has an important influence on the phase fractions and the final crystallographic texture of a hot-rolled dual-phase ferrite+martensite CMnCrSi steel. The final texture is influenced by three main microstructural processes: the recrystallization of the austenite, the austenite deformation, and the austenite-to-ferrite transformation. The amount of strain below T _nr plays a major role in the relative amounts of deformed and recrystallized austenite after rolling. Recrystallized and deformed austenite have clearly different texture components and, due to the specific lattice correspondence relations between the parent austenite phase and its transformation products, the resulting ferrite textures are different as well. In addition, austenite deformation textures result from either dislocation glide or the combination of dislocation glide and mechanical twinning, depending on the stacking fault energy (SFE). The texture components in hot-rolled dual-phase steels were studied by means of X-ray diffraction (XRD) measurements and orientation imaging microscopy (OIM). A clear crystallographic orientation difference was observed between the ferrite phase, transformed at temperatures near A _r3 , and the ferritic bainite and martensite phases, formed at lower temperatures. The results suggest that the primary ferrite, nucleated at temperatures close to A _r3 , transformed from the deformed austenite. The low-temperature constituents, bainite and martensite, form in the recrystallized austenite.     

Tempering of Mn and Mn-Si-V dual-phase steels

Changes in the yield behavior, strength, and ductility of a Mn and a Mn-Si-V d11Al-phase (ferrite-martensite) steel were investigated after tempering one hour at 200 to 600 °C. The change in yield behavior was complex in both steels with the yield strength first increasing and then decreasing as the tempering temperature was increased. This complex behavior is attributed to a combination of factors including carbon segregation to dislocations, a return of discontinuous yielding, and the relief of resid11Al stresses. In contrast, the tensile strength decreased continuously as the tempering temperature was increased in a manner that could be predicted from the change in hardness of the martensite phase using a simple composite strengthening model. The initial tensile ductility (total elongation) of the Mn-Si-V steel was much greater than that of the Mn steel. However, upon tempering up to 400 °C, the ductility of the Mn-Si-V decreased whereas that of the Mn steel increased. As a result, both steels had similar ductilities after tempering at 400 °C or higher temperatures. These results are attributed to the larger amounts of retained austenite in the Mn-Si-V steel (9 pct) compared to the Mn steel (3 pct) and its contribution to tensile ductility by transforming to martensite during plastic straining. Upon tempering at 400 °C, the retained austenite decomposes to bainite and its contribution to tensile ductility is eliminated. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

双相不锈钢及其特殊性能与应用

介绍双相不锈钢的基本概念、发展历史、特殊性能、类型、牌号、国内外生产情况及其广泛的应用领域．     

Acoustic emission during deformation of dual-phase steels

The study of acoustic emission (AE) during deformation of dual-phase steels consisting of ferrite (F) and pearlite (P)/martensite (M) indicates that the AE peak in the yielding region always appears at the beginning of the macroplastic deformation. After macroyielding starts, the AE decreases because the dislocation velocity, ν_d, decreases. The total AE energy, ∑E_lp, emitted during Lüders band propagation is related to the Lüders strain, ε_l and ∑E_lp/ ε_l, decreases as ε_l increases because of the increase in ε, resulting from the decrease in dislocation velocity, ν_d. After quenching from the two-phase region at cooling rates larger than a certain critical value, a second AE peak, which is produced by the cracking mainly at the interfaces between M and F and secondly in martensitic particles, appears in a certain plastic strain range in addition to the one in the yielding region. As the cooling rate becomes too fast, the AE peak in the yielding region disappears, and the second AE peak cannot be completed due to the brittle fracture.     

Self-organizing phenomena in bainite steels

Based on an analysis of existing data and our own research results, we made the following scientific hypothesis: a decrease in the lattice parameter for iron due to the effect of two factors — negative temperatures and silicon content — for t_test < t_Si and [Si] > 2.2% causes embrittlement of the iron (KCU = 0 J/cm²) due to dominance of covalent forces. By causing the regular occurrence of new covalent binding forces between iron atoms, silicon contents in excess of the [Si] > 0.50% threshold in iron and Fe-C bainite alloys give rise to subsequent self-organizing phenomena, the development of new bifurcations (abrupt increases in hydrogen solubility and in the amount of austenite in the alloy; formation of two supersaturated solid solutions 〈α+γ〉; occurrence and reversal of “rejuvenation”). This hypothesis provides scientific explanations not only for the processes described above, but also for processes related to graphitization, weldability, floccene formation, achievement of high strength, etc. This scientific hypothesis will become the basis for new approaches in existing areas, e.g., for development of iron-based hydrogen-storage alloys (HSAs).