Showing papers by "Kaneaki Tsuzaki published in 2012"

Effect of large strain cold rolling and subsequent annealing on microstructure and mechanical properties of an austenitic stainless steel

Abstract: The microstructural evolution of an S304H steel during bar rolling to a strain of 4 and subsequent annealing as well as its effect on the mechanical properties were investigated. The cold working was accompanied by a strain-induced martensitic transformation, leading to the development of lamellar-type microstructure consisting of highly elongated austenite/ferrite subgrains with a mean transverse size of approximately 50 nm; the austenite volume fraction was approximately 0.35. This material exhibited a yield strength above 2000 MPa. The subsequent annealing resulted in grain coarsening following the ferrite → austenite reversion, which led to almost full austenitization at temperatures above 700 °C. The formation of the austenite/ferrite lamellar structure that mixed with separate equiaxed grains occurred after annealing at temperatures of T ≤ 700 °C. The grain coarsening was accompanied by a degradation in strength, although the yield strength of above 1000 MPa remained after 2 h of annealing at 700 °C. The discontinuous recrystallization of austenite resulted in the development of a relatively coarse-grained microstructure at T ≥ 800 °C.

Hydrogen Embrittlement of a 1500-MPa Tensile Strength Level Steel with an Ultrafine Elongated Grain Structure

Abstract: A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500 °C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT and AACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500 °C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong 〈110〉//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prior-austenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample.

Heterogeneous twin formation and its effect on tensile properties in Ti–Mo based β titanium alloys

Abstract: The deformation modes and the tensile properties were investigated in a Ti–15Mo–5Zr alloy with different heat treatments. Both the {3 3 2}〈1 1 3〉 twinning and the dislocation slip occurred in the samples with the heterogeneous distribution of Mo and Zr atoms. On the other hand, the twinning disappeared when the elemental distribution became homogeneous. The high yield strength and significant uniform elongation resulted from the combination of the twinning and the slip, and the more significant work hardening rate was obtained by the enhancement of the twinning. The heterogeneous twin formation was mainly discussed based on the effects of the interval of heterogeneous elemental distribution and the grain orientation.

Premature Fracture Mechanism in an Fe-Mn-C Austenitic Steel

Abstract: We investigated the cause for poor ductility in austenitic Fe-Mn-C steels under a specific condition. Tensile tests were performed on an Fe-17Mn-0.3C steel at 273 K, 294 K, 323 K, 373 K, 423 K, 473 K, and 523 K (0 °C, 21 °C, 50 °C, 100 °C, 150 °C, 200 °C, and 250 °C). Microstructural observations were conducted by optical microscopy, atomic force microscopy, scanning electron microscopy and the X-ray diffraction method. e-martensitic transformation was concluded to be the major cause for the poor ductility. The cracks were initiated from the annealing twin boundaries that interacted with the e-martensite.

Hydrogen trapping phenomena in martensitic steels

Abstract: : This chapter begins with a brief description of hydrogen trapping theories and the major hydrogen trapping evaluation techniques including thermal desorption spectrometry (TDS). It then illustrates the hydrogen trapping characteristics of elemental martensitic microstructural features using alloys with a simple chemical composition and well-defined microstructure which maximize the effect of a microstructure feature of interest.

Quasi-cleavage Fracture along Annealing Twin Boundaries in a Fe–Mn–C Austenitic Steel

Abstract: Martensitic transformation from FCC (γ) to HCP (ε) is often observed in austenitic high Mn steels. For example, the ε-martensitic transformation occurs at cryogenic temperature in twinning induced plasticity steels1,2) and at room temperature in shape memory alloys.3,4) The ε-martensitic transformation produces the shape memory effect1) and high work hardening rates;5) however, it is also known to cause premature fracture resulting in tensile deformation. The fracture mode is known to be either an intergranular6,7) or quasi-cleavage2,6,7) fracture when this occurs. The premature fracture associated with the ε-martensitic transformation is reported to be caused by the interactions between ε-martensite plates and boundaries in Fe–27Mn–0– 7Si and Fe–20Mn–4Cr–0.5C–0.5Si alloys.6,7) The cracks are thought to be induced from the stress concentration arising from the interactions. However, the crack initiation sites of the premature fracture caused by ε-martensitic transformation are still unclear in ternary Fe–Mn–C austenitic steels which have recently been determined to be transformation induced plasticity (TRIP)8) and twinning induced plasticity (TWIP) steels.2,8–10) Primary ε-martensite plate,6) grain boundary,6,7) and twin boundary have been considered as possible boundaries that interact with ε-martensite for the occurrence of the premature fracture. We found that the annealing twin boundaries were the crack initiation sites in the Fe–17Mn–0.3C steel in this study and will provide the evidences.

Effects of lattice defects on indentation-induced plasticity initiation behavior in metals

Abstract: Plasticity initiation behavior that appears as a pop-in phenomenon on a loading process during indentation-induced deformation was investigated to reveal the effects of lattice defects such as grain boundary and solute element for various metallic materials including Fe alloys through instrumented nanoindentation techniques. The critical load Pc of pop-in on a loading process is lower in the vicinity of the grain boundary than in the grain interior, but the relative hardness of the boundary is equal to or greater than that in grain interior. In-solution Si produces a larger increase in the Pc for both the grain boundary and the grain interior in the Fe–Si alloy than in the interstitial-free steel. The maximum shear stress corresponding to the Pc underneath the indenter is directly proportional to the shear modulus in single crystals with various crystallographic structures. Microstructural effects on the Pc are considered based on some dislocation models.

Effect of Dislocation Density on the Initiation of Plastic Deformation on Fe­C Steels

Abstract: The effect of pre-existing dislocations and interstitial carbon on the initiation of plastic deformation in interstitial free (IF) steel and ultra low carbon (ULC) steel were investigated by the nanoindentation technique. The critical load, Pc, of the pop-in phenomenon, which corresponds to plasticity initiation, appeared clearly on the loading curve. The Pc in high dislocation density materials was smaller than that in low dislocation density materials with no difference between IF and ULC while the Pc in low dislocation density materials was remarkably higher in ULC than in IF. These results indicate that the interstitial carbon in the matrix does not affect the pop-in phenomenon when there are pre-existing dislocations or dislocation sources, and we discuss the reason for their occurrence occurs in high dislocation density materials.

Enhancement of Upper Shelf Energy through Delamination Fracture in 0.05 pct P Doped High-Strength Steel

Abstract: An ultrafine elongated grain (UFEG) structure with strong 〈110〉//rolling direction (RD) fiber deformation texture was produced by warm-caliber rolling at 773 K (500 °C) and final tempering at 823 K (550 °C), namely tempforming in the 1200 MPa-class, medium-carbon, low-alloy steel with phosphorus (P) content of 0.053 wt pct. Charpy impact tests and tensile tests were performed at a temperature range of 77 K (–196 °C) to 623 K (350 °C) on the tempformed (TF) samples along with a conventional quenched and tempered (QT) samples. The QT structure showed a low upper shelf energy of 70 J and a high ductile-to-brittle transition temperature (DBTT) of 373 K (100 °C) as a result of P segregation and intergranular fracture. A remarkable increase in the upper shelf energy to 150 J from 70 J and a low DBTT of approximately 103 K (–170 °C) were obtained in the UFEG structure. P segregation embrittlement disappeared completely in the UFEG structure, and ductile fracture on the planes normal to RD along with delamination fracture on the planes along RD were observed at a temperature range of 123 K (–150 °C) to 423 K (150 °C). The enhanced delamination occurred because of the microstructural anisotropy of the UFEG structure, a strong 〈110〉//RD fiber deformation texture, and interfaces (i.e. ferrite grain boundaries and cementite particles-ferrite matrix interfaces) weakened by P segregation as feasible crack propagation paths. We studied the delamination (crack-arrester-type) fracture in 0.053 pct P doped high-strength steel along with upper shelf energy and DBTT obtained from the UFEG structure.

Selective appearance of-martensitic transformation and dynamic strain aging in Fe-Mn-C austenitic steels

Abstract: The influence of stress-induced e-martensitic transformation on the serrated flow behavior associated with dynamic strain aging was investigated. The e-martensitic transformation was controlled by changing the deformation temperature and adding Si to Fe–17Mn–xSi–0.3 C alloys. The addition of Si promoted the e-martensitic transformation, and suppressed the slip deformation due to solution hardening. The initiation of serrations around room temperature was delayed by the promotion of e-martensitic transformation which initiated plastic deformation. The critical stress for the occurrence of serrations and the critical stress for the occurrence of slip deformation were found to have a linear relationship.

Enhanced upper shelf energy by ultrafine elongated grain structures in 1100MPa high strength steel

Abstract: The ultrafine elongated grain (UFEG) structure with strong //RD fiber deformation texture was produced by warm caliber-rolling at 500 °C and final tempering at 550 °C, namely tempforming in the 1100 MPa medium carbon-low alloy steel with ultralow phosphorus and sulfur concentrations. Charpy impact tests were performed at temperature range of −196 °C to 150 °C on the UFEG structure along with a conventional quenched and tempered (QT) structure. A remarkable increase in upper shelf energy of 150 J was obtained in the UFEG structure without delamination, while that of QT structure was 97 J. The UFEG structures tempered at higher temperatures of 625 and 700 °C showed remarkable increase in Charpy absorbed energy from 150 J to 187 J and 203 J. Also, the QT structure absorbed almost same energy as UFEG structure at 700 °C. The enhanced toughness was discussed with tensile ductility, void nucleation and growth, and their relations to microstructure including the //RD fiber deformation texture.

Influence of Dislocation Separation on Dynamic Strain Aging in a Fe­Mn­C Austenitic Steel

Abstract: ential slip system of trailing partials is not simply determined by Schmid factors of the trailing partials, but is also affected by the Schmid factors of the corresponding leading partials. In spite of these significant differences among the motions of perfect, leading partial and trailing partial dislocations, the effect of the separation of dislocations has not been considered for discussing the dynamic strain aging behavior in low stacking fault energy materials so far. In order to discuss and correlate the effect of the dislocation separation with experimental facts, we selected aF e­17Mn­0.3C alloy (mass%) as an austenitic steel with low stacking fault energy. The temperature and strain rate dependence of dynamic strain aging behavior was investigated. We found that the deformation temperature and the critical strain for the onset of serrations could not be expressed as a simple relationship, i.e., the critical strain decreased in the relatively-low temperature region increased in the middle range temperature region, and decreased again in the relatively-high temperature region with increasing deformation temperature. The characteristic dynamic strain aging behavior may have been caused by the separation of dislocations. This paper is intended to share the importance of the dislocation separation and its experimental facts in dynamic strain aging of low stacking fault energy materials.

Inverse grain size dependence of critical strain for serrated flow in a Fe–Mn–C twinning-induced plasticity steel

Abstract: The grain size dependence of critical strain for serrations associated with dynamic strain aging has been examined in a twinning-induced plasticity steel. Tensile tests were conducted at various deformation temperatures and strain rates in a Fe–17Mn–0.6C steel (mass%) with grain sizes 3.5, 10, 23, 37, and 44 µm. In addition, the carbon concentration varied from 0.3 to 0.8 in the Fe–17Mn–xC steels with coarse grains. The critical strain for the onset of serrations was found to show an inverse grain size dependence, i.e., the critical strain increased with the decrease in grain size, the opposite of what occurs in conventional alloys.

Enhanced uniform elongation by pre-straining with deformation twinning in high-strength β-titanium alloys with an isothermal ω-phase

Abstract: It is shown that pre-straining with deformation twinning is a novel approach to enhancing the uniform elongation of a high-strength β-type Ti–15Mo alloy (mass%) with isothermal ω-phase precipitation Pre-existent mechanical {3 3 2}⟨1 1 3⟩ twins hinder the early onset of plastic instability (necking) after yielding, which is often caused by the presence of the isothermal ω-phase, and enhance the uniform elongation markedly from 0% to 13% at a yield strength level of 900 MPa

Strength evaluation of α and β phases by nanoindentation in Ti–15Mo alloys with Fe and Al addition

Abstract: The strengths of the α precipitate and the β matrix were evaluated by nanohardness in the Ti−15Mo−1Fe and Ti−15Mo−3Al alloys and compared to those of the Ti−15Mo alloy. The α phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the β phase and depleted in the α phase, while Al was enriched in the α phase and depleted in the β phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti−15Mo−1Fe and Ti−15Mo−3Al alloys than in the Ti−15Mo alloy. The nanohardness measurements indicated that the α phase was softer than the β phase in both Ti−15Mo−1Fe and Ti−15Mo alloys, while it was harder in the Ti−15Mo−3Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched β phase in the Ti−15Mo−1Fe alloy and by the strength of the...

Influence of Processing Regimes on Fine-Grained Microstructure Development in an Al-Mg-Sc Alloy by Hot Equal-Channel Angular Pressing

Abstract: Grain refinement during equal-channel angular pressing (ECAP) was studied in a commercial Al­5.8%Mg­0.3%Sc alloy at temperatures from 473 to 723K (³0.5­0.8Tm). The samples were quenched in water in every ECAP pass, which is a conventional cyclic process. ECAP to ¾ = 12 resulted in ultrafine-grained structures developed uniformly at high strains at 473 and 723K, while processing at 573K led to the evolution of a duplex grain structure containing partially much coarser grains. In contrast, ECAP process continuously carried out to ¾ = 12 without interruption at 573K, resulted in development of a uniform ultrafine-grained structure. Effects of processing regimes on microstructural evolution in the Al­Mg­Sc alloy are discussed. [doi:10.2320/matertrans.MD201108]

Effect of deformation temperature on tensile properties in a pre-cooled Fe–Mn–C austenitic steel

Abstract: We investigated the tensile deformation behavior of a Fe–17Mn–0.3C (wt%) steel containing thermally-induced HCP-martensite that was formed by cooling to 77 K beforehand from various temperatures. In the temperature range where deformation-induced HCP→FCC reverse transformation and deformation twinning occur, the yield strength was enhanced by the pre-existing HCP-martensite, and the brittle cracking associated with the HCP-martensite was suppressed by the deformation-induced HCP→FCC reverse transformation after yielding. Additionally, the work hardening was sustained by deformation twinning. As a result, the yield and flow stresses were enhanced without any loss in elongation at the specific temperatures that were used in this study.

Dynamic recrystallization mechanisms operating under different processing conditions

Abstract: Dynamic recrystallization (DRX) is one of the most important mechanisms for microstructure evolution during deformation of various metals and alloys. So-called discontinuous DRX usually develops in structural materials with low to medium stacking fault energy during hot working. The local migration, i.e. bulging, of grain boundaries leads to the formation of recrystallization nuclei, which then grow out consuming work-hardened surroundings. The cyclic character of nucleation and growth of new grains during deformation results in a dynamically constant average grain size. The dynamic grain size is sensitively dependent on temperature and strain rate and can be expressed by a power law function of flow stress with a grain size exponent of about-0.7 under conditions of hot working. Recent studies on DRX phenomenon suggest that a decrease in deformation temperature changes the structural mechanism for new grain formation. As a result, the grain size exponent in the relationship between the dynamic grain size and flow stress approaches about-0.25 under warm working conditions.

Ultrafine Grain Evolution in Austenitic Stainless Steel during Large Strain Deformation and Subsequent Annealing

Abstract: The structural changes in a 304-type austenitic stainless steel during large strain cold rolling and subsequent annealing were studied. The severe deformation resulted in the development of highly elongated grains/subgrains aligned along the rolling axis. The transverse grain/subgrain size rapidly decreased to its minimal value of about 50 nm at relatively small strains of ~1 and then hardly changed upon following deformation. Such a structural response on cold working was associated with multiple twinning resulting in fast grain subdivision. The processing was accompanied by a partial martensitic transformation resulting in a decrease of austenite volume fraction to about 0.35 after straining to e = 4.0. Isochronal annealing for 30 min was characterised by a gradual coarsening of grains, the average size of which increased to about 200 nm after heating to 800°C. The high elongation of ferrite grains facilitated simultaneous homogeneous nucleation of austenite grains throughout the matrix upon heating; and, therefore, promoted the development of ultrafine grained structure with the size of structural elements well below 1 micron.

Fe-mn-c-based twip steel having remarkable mechanical performance at very low temperature, and preparation method thereof

Abstract: Provided is a Fe—Mn—C-based twinning-induced plasticity (TWIP) steel which includes 13 wt % to 24 wt % of manganese (Mn), 0.4 wt % to 1.2 wt % of carbon (C), and iron (Fe) as well as other unavoidable impurities as a remainder, is manufactured by caliber rolling, has a microstructure including elongated grains that are elongated in a rolling direction, and has an average grain size of the elongated grains in a direction perpendicular to the rolling direction of 1 μm or less.