土田 紀之

The effects of pre-strain on the mechanical properties of high-strength martensitic steels were investigated using either strain tempering (ST) or quenching and tempering (QT) samples. In the tensile tests at deformation temperatures between 296 and 573 K, the ST sample exhibited an increase in both the tensile strength (TS) and uniform elongation (U.El) at 473 to 523 K, whereas the QT sample showed an increase in U.El with little change in the TS and yield strength (YS). The results of in situ neutron diffraction experiments revealed an increase in the stress partitioning to the bcc phase with an increase in the deformation temperature from 296 to 523 K. The difference in the phase stress between the bcc and cementite phases decreased with an increase in the temperature due to the decrease in the cementite strength. Pre-strain of 0.5% increased the YS at 296 K with a slight work hardening; the initial dislocation density (ρ) decreased at 523 K, but increased significantly after yielding, leading to a better combination of TS and U.El. The combination of pre-strain, tempering, and deformation temperatures caused the change in ρ before deformation and the increase in ρ after yielding of the martensitic steel.

Deformation-induced martensitic transformation (DIMT) during tensile or compressive deformations of the bainitic steels with various carbon content (0.15%C, 0.25%C, 0.62%C) was studied. The initial microstructure before the deformation tests was prepared by the austempering at 400°C to obtain bainitic structure consisting of bainitic ferrite and retained austenite. The volume fraction of the retained austenite was larger in the bainitic steel with the larger carbon content. In all of the bainitic steels, the tensile deformation exhibited larger work hardening than the compression. This difference indicates the suppression of the DIMT at the compression, and actually the measurements of electron back scattering diffraction (EBSD) confirmed the less reduction of retained austenite at the compression of all the bainitic steels. Additionally, the steel with the highest carbon content was examined by in situ neutron diffraction and clarified the difference similar to that obtained by the EBSD measurement. The regression of the relation between the fraction of austenite and applied strain with the conventional empirical equation revealed that the kinetic of DIMT is strongly dependent with the stress polarity, but not significantly changed by the carbon content. The mechanism of the DIMT dependence of the stress polarity was discussed with the deformation texture and the crystallographic orientation dependence of DIMT.

Effects of pre-strain and tempering on mechanical properties in high-strength martensitic steels were investigated. In this study, strain tempering (ST) and quenching and tempering (QT) martensitic steels were prepared, and their mechanical properties were studied. In the tensile tests at the deformation temperatures between 296 and 573 K, the ST sample increased both of tensile strength (TS) and uniform elongation (U.El) from 473 to 523 K whereas the QT sample increased U.El with little change of TS. From the in situ neutron diffraction experiments, stress partitioning to the bcc phase increased with an increase in the deformation temperature from 296 to 523 K. The difference of phase stress between the bcc and cementite phases decreased with increasing the temperature because of a decrease in the cementite strength. In the ST sample, Pre-straining of 0.5% increased YS at 296 K with slight work hardening. The initial value of dislocation density (ρ) decreased at 523 K but ρ increased significantly after yielding, leading to better combination of TS and U.El. The combinations of pre-strain, tempering, and deformation temperatures have changed ρ before deformation and the increase of ρ after yielding of the martensitic steels.

This study investigated the true stress (σ)–true strain (ε) relationship up to the plastic deformation limit in ferrite–pearlite (FP) steel at various temperatures. We found that a decrease in the deformation temperature resulted in an increase in the lower yield strength (LYS) and the tensile strength (TS), and a decrease in both the uniform and the total elongations. When the temperature was increased from 673 to 773 K, the TS increased and the uniform and total elongations decreased due to the blue brittleness. In the σ–ε relationship up to the plastic deformation limit of the FP steel at various temperatures, ε at the plastic deformation limit increased with an increase in the temperature and was correlated with the reduction of cross-sectional area of tensile specimen. Microcracks were observed in the cementite plates as well as tearing in the pearlite lamellae. The temperature dependence of fracture stress (σ_Z) was small. In particular, the values of σ_Z in the temperature range of ductile fracture was almost the same, and is applicable to the condition of the plastic deformation limit. When the ratio of LYS to σ_Z was 0.5, the area fraction of brittle fracture was 50%.

<p>In situ neutron diffraction measurements of two low-alloy steels and a 304-type stainless steel during tensile and creep tests were performed at room temperature. Changes in the diffraction pattern, the integrated peak intensities of austenite (γ), and the peak positions of γ were analyzed and discussed to elucidate the relationship between intergranular stress in γ and the occurrence of martensitic transformation during deformation. Tensile loading experiments revealed that the susceptibility to martensitic transformation depended on the γ-(hkl) grains, where γ-(111) grains underwent martensitic transformation at the latest. The volume fractions of γ were found to decrease under an applied load but to remain almost unchanged under constant load in creep tests, where the lattice strains of γ-(hkl) grains were mostly unchanged. The γ-hkl dependence of the susceptibility to martensitic transformation was found to be controlled by the shear stress levels in γ-(hkl) grains, which were affected by the intergranular stress partitioning during deformation.</p>

High-speed deformation behavior and strain rate dependency of mechanical properties of 1 GPa-grade TRIP-aided multi-phase (TRIP) steels were studied. The strain rate range in this study was between 3.3 × 10-6 and 103 s-1, and the effect of retained austenite (R) shapes on TRIP effect in the 1 GPa-grade TRIP steel was also focused on. The effects of strain rate on tensile strength and flow stress in the TRIP steels were small whereas that on uniform elongation was large. The strain rate dependencies of tensile strength and uniform elongation in the TRIP steels were more closely to those of the metastable austenitic stainless steels than the conventional TRIP and dual-phase steels. The 1 GPa-grade TRIP steel with theR shape of needle-like showed better tensile properties and absorbed energy in the present strain rate range. The volume fraction of R more than 20% and the matrix microstructure of martensite seem to be important factors in the high-strength TRIP steels.

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 (gamma). 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 gamma were similar regardless of different carbon contents. The phase stress of martensite was found much larger than that of gamma 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 gamma decreasing.

This study investigated room-temperature creep tests using a low-carbon TRIP steel to clarify the TRIP effect on tensile deformation behavior under constant load. Nominal strain and strain rate increased with an increase in applied stress and those were almost stagnated at holding times of about 3.0 x 10(4) s. The volume fractions of deformation-induced martensite (alpha') at a given true strain obtained from constant load creep tests were larger than those from tensile tests. From the in situ neutron diffraction experiments during the constant load creep tests, the phase strain of the austenite (gamma) phase in the creep tests was found to be larger than that in the tensile tests at the same applied stress. This means that the phase strain or true stress of they phase in the TRIP steel was associated with the difference in the volume fraction of alpha' between the creep and the tensile tests. The in situ neutron diffraction experiments also showed that the lattice strains of the TRIP steel in the constant load creep tests were independent of &lt; hkl &gt; grains in gamma and alpha phases.

<p>The effect of specimen size on tensile deformation behavior of the ultrafine-grained ferrite-cementite (FC) steel was studied from the viewpoint of true stress (σ)-true strain (ε) relationship. In this study, we took notice of the estimation of σ-ε relationship up to the plastic deformation limit in order to discuss tensile deformation behavior up to fracture, especially in the local deformation behavior which the size dependence on mechanical properties is large. From the ultrafine-grained FC steel, round tensile test specimens with various sizes of gage length (GL) and gage diameter (φ) were prepared. Static tensile tests and stepwise tensile tests were performed with an initial strain rate of 5.0 × 10^-4 s^-1 at 296 K using a gear-driven-type machine. In the nominal stress-strain curves, the lower yield strength and total elongation increased with a decrease in the gage length. The σ-ε relationship up to the plastic deformation limit was almost independent of the specimen size, whereas the true strain at the plastic deformation limit and reduction of area decreased when the value of GL/φ became less than 1. The σ-ε relationship up to the plastic deformation limit estimated by Bridgman equation is unique data independent of specimen size and is effective to discuss the effect of specimen size on deformation behavior.</p>

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: 49.6 mu m) 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 mu m) had few grains with twinning even after the tensile deformation. The electron back scattering diffraction (EBSD) 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.

Effects of temperature and strain rate on tensile properties in a lean duplex stainless steel S32101 were investigated. The 0.2% proof stress and tensile strength increase with a decrease in the temperature and an increase in the strain rate. The uniform elongation decreased with an increase in the strain rate. In the temperature dependence on uniform elongation, it increased from 273 K to 283 K and indicated the maximum uniform elongation at 258 K. This is closely associated with TRIP effect because austenite is transformed to stress-induced martensite at temperatures below 283 K from the x-ray diffraction experiments. The stress-induced transformation behavior at 258 K, at which the maximum uniform elongation was obtained, had things in common with the case of metastable austenitic stainless steels. When the tensile properties were compared between the S32101 and the metastable austenitic stainless steels, the increase in the uniform elongation due to TRIP effect was almost the same. At low temperatures below about 250 K, the uniform elongations of the metastable austenitic steels were smaller than that of the S32101 because of the large amount of stress-induced martensite at small strains.

The effect of the Ni equivalent (Ni-eq.) on the transformation-induced plasticity (TRIP) effect in metastable austenitic steels was investigated to clarify the conditions of stress-induced martensitic transformation behavior for maximum uniform elongation. Results of static tensile tests showed that the tensile strength increased with decreasing Ni-eq. and that uniform elongation reached a maximum value at the Ni-eq. of 23.7%. The volume fraction of martensite at the same true strain increased as the Ni-eq. decreased. The conditions under which the stress-induced transformation resulted in the maximum uniform elongation due to the TRIP effect in the metastable austenitic steels were summarized in terms of the volume fraction of stress-induced martensite and the rate of transformation, which were almost coincident with previous results. (C) 2011 Elsevier B.V. All rights reserved.

Deformation behaviors of two TRIP-type multiphase steels with different carbon contents were studied by in situ neutron diffraction measurement during tensile deformation at RT. The tensile test was conducted in a step-load controlling manner during the elastic region, and in a continuous manner with a constant crosshead speed by an initial strain rate of 1.8×10-5 s-1 during the plastic region. Austenite grains were observed to bear higher phase stresses than ferrite grains in both steels. Austenite peak intensities started to decrease at the onsets of plastic deformation in both steels, showing the occurrence of stress induced martensitic transformations. Martensite peaks were carefully analyzed to estimate phase strains, and as the results martensite grains were found to bear largest phase stresses during plastic deformation. © 2013 Materials Research Society.

To investigate the tensile deformation behavior of a lean duplex stainless steel (S32101) from the viewpoints of plastic deformability among phases or grains, we performed static tensile tests, in situ neutron diffraction, and white x-ray diffraction experiments at room temperature. In the static tensile tests, the S32101 steel displayed a larger uniform elongation and a better tensile strength-uniform elongation balance than a commercial SUS329J4L duplex stainless steel. A larger uniform elongation of S32101 is associated with the macroscopic work hardening behavior that a work hardening rate higher than the flow stress can maintain up until high true strains. From the experimental results of synchrotron radiation white x-ray diffraction experiments, the hard phase of S32101 was changed from the ferrite (α ) phase to austenite (γ ) one during tensile deformation. This led to a larger stress partitioning between the phases at the latter stage of deformation. From the experimental results of in situ neutron diffraction, it was found that the stress partitioning of the γ phase in the S32101 was the largest among the present results. Therefore, the larger work hardening rate of S32101 can be explained by the large stress partitioning of the ã phase, that between ã and á phases and ã volume fraction. © 2013 ISIJ.

Effects of temperature and strain rate on tensile properties in metastable austenitic stainless steel SUS301L were studied in order to clarify the conditions of stress-induced martensitic transformation behavior for the maximum uniform elongation through the TRIP effect. The experimental results of the previously studied SUS304 steel were used to compare the conditions of metastable austenitic steels with different austenite stability. In the static tensile tests for the SUS301L steel at temperatures between 123 K and 373 K, the tensile strength increased with decreasing temperature, and uniform elongation reached a maximum at 323 K. The volume fraction of stress-induced martensite (Vα) at the same true strain increased with a decrease in temperature. For the strain-rate dependence on transformation kinetics, Vα decreased at strain rates higher than about 10-2 s-1 due to the temperature rise caused by adiabatic deformation. The equation for stress-induced transformation behavior proposed by Matsumura et al. was modified to consider the saturation value of stress-induced martensite, and the modified equation could describe the transformation kinetics precisely. The conditions of stress-induced transformation for the maximum uniform elongation through the TRIP effect were coincident between the SUS301L and the SUS304 with different austenite stability: the volume fraction of martensite at a true strain of 0.3 is approximately 5% and the maximum transformation rate is almost 2 at a higher true strain near uniform elongation. © 2013 ISIJ.

True stress (sigma)-true strain (epsilon) relationships until just before fracture, i.e., the plastic deformation limit, were estimated by the stepwise tensile test and the Bridgman equation for various metals and alloys with different crystal structures. The estimated sigma-epsilon relationships were different from the nominal stress-strain curves including the conventional tensile properties. In the relationships between the true stress (sigma(pdl)) and true strain (epsilon(pdl)) at the plastic deformation limit, SUS304 and SUS329J4L indicated a better sigma(pdl)-epsilon(pdl) balance. On the other hand, SUS329J4L, tempered martensite, and an ultrafine-grained steel showed superior results in the yield strength-epsilon(pdl) balance. The estimated sigma-epsilon relationship for the ultrafine-grained steel suggests that grain refinement strengthening can improve sigma and epsilon up until the plastic deformation limit. The value of epsilon(pdl) became larger with increasing the reduction in area and a decrease in the fracture stress. The products of sigma(pdl) and epsilon(pdl) became larger with increasing work-hardening rate at the plastic deformation limit.