Noriyuki Tsuchida

In situ neutron diffraction study of IF and ultra low carbon steels upon tensile deformation

Abstract Tensile behavior of an interstitial-free (IF) and an ultra low-carbon (ULC) steel bearing similar concentrations of carbon and nitrogen was studied by means of an in situ neutron diffraction technique. The (110), (200) and (211) lattice plane strains were determined as a function of the applied stress, revealing three deformation stages; (1) elastic deformation, (2) grain to grain yielding and (3) the stage III deformation. The microstrain associated with dislocation density (ρ) was increased with tensile straining and the increasing rate was higher in the ULC steel than in the IF steel, resulting in higher flow stress in the ULC steel due to dynamic strain aging. The estimated ρ was summarized as a function of strain (e); ρ = ρ0 + AeB, leading to the description of flow stress (σ); σ=120+0.89μb A1/2eB/2 MPa, where ρ0, A and B were constants, μ shear modulus, and b the magnitude of Burgers vector. The above constants were influenced by dynamic strain aging through the Bailey–Hirsch relation.

Application of the Kocks–Mecking model to tensile deformation of an austenitic 25Cr–19Ni steel

Abstract Stress–strain relationships obtained by tensile test below room temperature for an austenitic 25Cr–19Ni steel were analyzed by using the Kocks–Mecking model to make clear the effects of temperature and strain rate on flow stress. A temperature range used here is between 77 and 296 K, a strain rate range between 10−9 and 10−2 s−1 and true strain below 0.2, where structure evolution depends on strain but scarcely on temperature and strain rate. This means that work-hardening rate is almost independent of test temperature and strain rate in the above ranges. Crosshead-arresting tests were performed to obtain flow stresses at 10−9 s−1 and the results suggested that the athermal stress could hardly be determined from the measurement of stress relaxation behavior at low temperatures. Flow curves obtained by the above deforming conditions are successfully described by using the Kocks–Mecking model with minor modifications. That is, we have claimed that the work-hardening consists of the thermal stress and the athermal stress. It should be noted that the flow curves for as hot-rolled specimens and for annealed specimens can be well simulated by changing the athermal stress.

A micromechanic modeling for transformation induced plasticity in steels

Abstract Transformation induced plasticity (TRIP) has been computed by using a micromechanic model, where the Eshelby inclusion theory, the Mori–Tanaka mean field theory and the Weng secant method are combined. This model enables us to understand experimental findings in three types of steels systematically, including (1) the influence of test temperature and strength of stress-induced martensite on elongation in metastable austenitic steels; (2) poor enhancement of elongation by TRIP of retained or precipitated austenite in Ni steels for cryogenic temperature use; and (3) the attractive combination of strength and ductility in Si bearing high strength sheet steels for automobile use that have currently been developed. A question why a small amount of austenite is effective to enhance the ductility in the last steels is clearly demonstrated by the present computations, where the effect of martensite strength for work-hardening is emphasized.

High Speed Deformation of Ultrafine Grained TWIP Steel

Tensile properties of twinning induced plasticity (TWIP) steels (31%Mn-3%Al-3%Si-Fe) with various mean grain sizes ranging from ultrafine grain size (1.1μm) to conventional one (35.5μm) at a wide range of strain rates from 10-3sec-1 to 103sec-1 were studied. The ultrafine grained TWIP steel exhibits a large work hardening and keeps an adequate elongation at any strain rate. The strength held to the Hall-Petch relationship at each strain rate and the Hall-Petch slopes do not change largely.

Martensite phase stress and the strengthening mechanism in TRIP steel by neutron diffraction

Two TRIP-aided multiphase steels with different carbon contents (0.2 and 0.4 mass%) were analyzed in situ during tensile deformation by time-of-flight neutron diffraction to clarify the deformation induced martensitic transformation behavior and its role on the strengthening mechanism. The difference in the carbon content affected mainly the difference in the phase fractions before deformation, where the higher carbon content increased the phase fraction of retained austenite (γ). However, the changes in the relative fraction of martensitic transformation with respect to the applied strain were found to be similar in both steels since the carbon concentrations in γ were similar regardless of different carbon contents. The phase stress of martensite was found much larger than that of γ or bainitic ferrite since the martensite was generated at the beginning of plastic deformation. Stress contributions to the flow stress were evaluated by multiplying the phase stresses and their phase fractions. The stress contribution from martensite was observed increasing during plastic deformation while that from bainitic ferrite hardly changing and that from γ decreasing.

Effect of initial grain size on inhomogeneous plastic deformation and twinning behavior in high manganese austenitic steel with a polycrystalline microstructure

The grain size effect on the deformation twinning in a high manganese austenitic steel which is so-called TWIP (twining induced plastic deformation) steel was studied in order to understand how to control deformation twinning. The 31wt%Mn-3%Al-3% Si steel was cold rolled and annealed at various temperatures to obtain fully recrystallized structures with different mean grain sizes. These annealed sheets were examined by room temperature tensile tests at a strain rate of 10-4/s. The coarse grained sample (grain size: 49.6μm) showed many deformation twins and the deformation twinning was preferentially found in the grains in which the tensile axis is parallel near to [111]. On the other hand, the sample with finer grains (1.8 μm) had few grains with twinning even after the tensile deformation. The electron back scattering diffraction (EB SD) measurements clarified the relationship between the anisotropy of deformation twinning and that of inhomogeneous plastic deformation. Based on the EBSD analysis, the mechanism of the suppression of deformation twinning by grain refinement was discussed with the concept of the slip pattern competition between the slip system governed by a grain boundary and that activated by the macroscopic load.

Tensile Deformation Behaviors of Metastable Austenitic Stainless Steels Studied by Neutron Diffraction

Tensile deformation behaviors of three austenitic stainless steels, JIS-SUS310S, 304 and 301L, were studied by static tensile tests and in situ neutron diffraction. In the mechanical properties obtained by the static tensile tests, the 304 and 301L steels showed better balance of tensile strength and uniform elongation than the 310S one because of TRIP effect. The angular dispersion neutron diffractions with a wavelength of 0.16 or 0.182 nm were performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency. The lattice plane strain, stress-induced martensite volume fraction, dislocation density and so on were estimated by the profile analysis as a function of applied stress. The change in lattice plane spacing for austenite indicated four deformation stages. In the comparison of lattice plane strain among the tested steels, a phase stress caused by the stress-induced martensite seems to overlap the intergranular stress of austenite phase. Judging from the results of profile analysis, the strain partitioning of austenite phase in metastable austenitic steels became larger with increasing of the volume fraction of stress-induced martensite during tensile deformation.

TRIP Effect in a Constant Load Creep Test at Room Temperature

In order to investigate TRIP (transformation induced plasticity) effect in different deformation style, a room temperature creep test under the constant load was conducted by using a TRIP-aided multi-microstructure steel. As a result, the volume fraction of deformation-induced martensite in the constant load creep test was larger than that in the tensile test. In situ neutron diffraction experiments during the constant load creep test were performed to discuss its reason. It is found from the in situ neutron diffraction experiments during the constant load creep tests that the phase strain of the austenite phase in the creep tests was larger than that in the tensile tests at the same applied stress.

Grain Refinement of Metal Surface Using Hot Shot Peening

In this study, the grain refinement near the surface of metal workpiece using hot shot peening was investigated to improve the surface properties of the workpiece. In this process, the grains were refined due to plastic deformation generated by the collision of a lot of shots under hot working conditions. A model experiment using two shots was carried out to examine the effects of the amount of deformation, the processing temperature and the time interval of the collision on grain size. In the experiment, the workpieces were stainless steel SUS304 and commercially pure copper. It was found that the global surface layer successfully attained to the fine grains by means of hot shot peening.

Metal Injection Molding of Nickel-Free Austenitic Stainless-Steels I-Manufacturing Process

Ni-free austenitic steels containing high nitrogen have been developed to protect against earth resource. High nitrogen steels (HNS) have a lot of advantages, e.g., HNS have high strength, corrosion resistance, toughness, work hardening rate and large rocking parameter in the Hall-Petch equation. On the other hand, it is difficult to fabricate HNS by IM method under 0.1 MPa and to work at room temperature. We have tried to make HNS by combined use of metal injection molding method (MIM) and nitrogen absorption method. Powder compositions used was Fe-17Cr-12Mn-3Mo.The benefit of this method is to make metal parts in near net shape. In order to use this method, we should know the sintering heat schedule, timing for introducing nitrogen gas, gas pressure and setter material etc. Therefore, the shrinkage rate, density and the solution-treated microstructure of MIM compacts were examined to find out the optimum conditions.