Papers by Author: Setsuo Takaki

Abstract: Quenching and partitioning (Q&P) treatment was applied to a commercial low carbon martensitic stainless steel, AISI Type 410 (Fe-12Cr-0.1C). The condition of partial quenching and partitioning was optimized with consideration of the untransformed austenite fraction and stability of austenite (carbon concentration in solid solution). As a result, the amount of retained austenite could be increased up to approximately 15 vol%. Tensile testing revealed that the specimens after Q&P heat treatment exhibited lower yield stress and larger work hardening rate compared with quench-and-tempered (Q&T) specimens under the same tensile strength level, resulting in a significantly better strength-ductility balance. It was confirmed that the TRIP effect had contributed to the mechanical property.

Abstract: The work hardening behavior was investigated in ferritic steels containing hard particles or soft Cu particles with various volume fractions and particle diameters, and then the effect of plastically deformable soft particles on the work hardening was evaluated in terms of the accumulation of GN dislocations. The amount of work hardening and dislocation density increased with an increase of volume fraction of dispersion particles and a decrease of particle diameter in hard particle dispersion steel. On the other hand, in soft Cu particle dispersion steel, the effect of volume fraction and particle diameter on work hardening behavior was relatively small. TEM observation suggested that stress relaxation around particle takes place by plastic deformation of Cu particle itself. In order to consider the effect of plastic deformation of Cu particles on accumulation of GN dislocations, "particle plastic accommodation parameter" was proposed to modify the Ashby's work hardening theory. As a result, the amount of work hardening was successfully predicted for both the hard and soft particle dispersion steels

Abstract: In ferritic stainless steels, the amount of Cr is moderately controlled to have good corrosion resistance in applied environment. However, it also affects the yield strength of ferritic stainless steels through solid solution strengthening and grain refinement strengthening. Until now, some researches have been performed using commercial stainless steels but the obtained results contain the effect of solute interstitials (C and N). In this paper, the influence of Cr on the above both strengthening mechanism was discussed by using interstitial free ferritic stainless steel in which carbon and nitrogen are completely fixed as Ti(C,N). A previous paper has reported that the addition of chromium gives different influences to the Hall-Petch coefficient depending on the amount of Cr. However, our research has reveals the fact that the change of Hall-Petch coefficient is not due to the effect of chromium but due to small amount of carbon which exists as an impurity in ferritic stainless steels. It was concluded that chromium itself does not give any influence to the Hall-Petch coefficient of ferritic iron.

Abstract: Yield strength of ferritic steel increases with grain refinement standing on the Hall-Petch relation. In low carbon ferritic steels, the following relation is established between yield strength σ_y and grain size d: σ_y [MPa]= 100+600/√d [μm]. The Hall-Petch coefficient of interstitial free steels is substantially small as 0.15MPa·√m but it can be greatly increased by the existence of small amount of solute carbon less than 60ppm. As for the effect of substitutional elements such as Cr and P, some papers reports fairly large influence to the Hall-Petch coefficient of ferritic iron. However, the effect of small amount of carbon is sometime neglected or not cleared on the evaluation of Hall-Petch coefficient in ferritic steels. In order to evaluate the effect of substitutional elements, the research should be performed using interstitial free steels to eliminate the influence of solute carbon and nitrogen. In this paper, Hall-Petch relation was examined in iron, Fe-Cr alloys and Fe-P alloys with 0.02-0.05mass% Ti and the following results were obtained: 1) The Hall-Petch coefficient of interstitial free iron is about 0.15MPa·√m. 2) Chromium does not give any influence to the Hall-Petch coefficient of ferritic iron, although the friction stress σ₀ is enhanced in proportional to chromium content (Δσ₀ [MPa]=7×(mass%Cr)). 3) Phosphorus does not affect the Hall-Petch coefficient of ferritic iron or reduce it somewhat but increases markedly the friction stress σ₀ (Δσ₀ [MPa]=250×(mass%P)^1/2). 4) Even under the co-existence of carbon with chromium and phosphorus, carbon dominantly works to increase the Hall-Petch coefficient of ferritic steels, but it is changeable due to the interaction between carbon and the other substitutional elements.

Abstract: Deformation-induced martensite preferentially nucleates at the twin boundary between matrix austenite and deformation twin in 316 type stainless steel. In the cold-rolled specimen, the martensite formed at the twin boundary has K-S relationships with both of the austenite matrix and the deformation twin, that is, “double K-S relationship” is realized. While in the case of cold-drawn specimen, two kinds of twins with different twin planes are typically observed, and therefore, the deformation-induced martensites are formed at the intersections of the two deformation twin boundaries, satisfying “triple K-S relationship” among austenite matrix and two deformation twins, although there is a small misfit from the perfect K-S relationship. The complicated crystallographic orientation relationship leads to a strong variant restriction for deformation-induced martensites. Due to the difference in the number of nucleation sites, martensitic transformation is greatly promoted in cold-drawn specimen rather than cold-rolled one.

Abstract: Yielding of polycrystalline low carbon steel is characterized by a clear yield point followed by unstable Lüders deformation and such a yielding behavior is taken over to fine grained steel with the grain size of 1μm or less. Yield strength of ferritic steel is increased with grain refinement standing on the Hall-Petch relation. The following equation is realized up to 0.2μm grain size in the relation between yield strength y and grain size d: y [MPa]= 100+600×d[μm]-1/2. In low carbon steel, it might be concluded that the Hall-Petch coefficient (ky) is around 600MPa•μm1/2. However, the ky value of interstitial free steels is substantially small as 130-180MPa•μm1/2 and it can be greatly increased by a small amount of solute carbon less than 20ppm. It was also cleared that the disappearance of yield point by purifying is due to the decrease in the ky value. On the other hand, the ky value is changeable depending on heat treatment conditions such as cooling condition from an elevated temperature and aging treatment at 90°C. These results suggest the contribution of carbon segregation at grain boundary in terms of the change in the ky value. On the contrary, substitutional elements such as Cr and Si do not give large influence to the ky value in comparison with the effect by carbon.

Abstract: The Fe-25Cr-1N alloy produced by solution nitriding possesses extremely high yield strength owing to the solid solution strengthening by nitrogen. However, it was found that the steel exhibited an insufficient elongation because of the brittle intergranular fracture caused during the uniform tensile deformation. This is due to the marked stress concentration at grain boundaries, which is derived from the grain coarsening caused during long time solution nitriding and the development of planar dislocation structure characteristic of high nitrogen austenitic steels. The most effective way to reduce the stress concentration at grain boundary during deformation should be grain refinement. In this study, grain refinement was attempted by the two-step heat treatment for the Fe-25Cr-1N(-Mn) alloy, and then the mechanical properties were investigated by means of tensile tests and fatigue tests. The two-step heat treatment resulted in the grain refinement of austenite to 20 microns in diameter. The intergranular fracture was greatly suppressed from 70% (as-solution-nitrided) to 10% (grain-refined) in area fraction by the grain refinement. In addition, elongation was markedly increased with local necking. The yield stress and tensile strength were also increased, and thus, the fatigue limit is also raised by more than 30%.

Abstract: The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

Abstract: In IF steel without interstitials, the Hall-Petch coefficient (ky) is low as about 100 MPa･μm1/2 but it is enhnced by adding some alloying elements. Author et al. has already reported that a small amount of carbon increases ky greatly while nitrogen does not give influence so much. On the other hand, it is already known that phosphorus has a tendency to segregate at grain boundary of polycrystalline ferritic iron and that phosphorus also gives some influence to ky. However, the effect of carbon is not considered on the evaluation of ky in steels containing phosphorus. In this study, effect of phosphorus on the ky of ferritic iron was investigated by varying the grain size in Ti bearing IF-steel with different amount of phosphorus. Besides, the concentration of phosphorus and carbon at grain boundary was estimated by the grain boundary segregation model proposed by Seah et al. and the interaction between phosphorus and carbon was discussed in connection with the change of ky.

Abstract: Grain size dependence of yield strength was reviewed for polycrystalline ferritic iron and low carbon steel. Yielding of polycrystalline low carbon steels was characterized by a clear yield point (upper yield point) and such a yielding behavior is taken over to ultra fine grained steel with the grain size below 1m. Yield strength (y) of polycrystalline low carbon steels obeys the Hall-Petch relation: y[MPa]=+600×d[m]-1/2 . The Hall-Petch coefficient ky is around 600 MPa･m1/2 for the commercial low carbon steels but it is lowered to about 100 MPa･m1/2 for interstitial fee steel. Besides, it is known in industrial pure iron (Fe-30ppmC) that ky increases with aging at 363K. The value of ky is also increases with increasing the amount of solute carbon content. The ky is enlarged from 100 MPa･m1/2 to 550 MPa･m1/2 by adding 60ppm of solute carbon and then levels off at around 600 MPa･m1/2 in the carbon concentration region above 60ppm. On the other hand, nitrogen hardly influences the ky value. Difference between C and N in the contribution to ky is probably due to the difference in grain boundary segregation behavior. Macroscopic yielding of polycrystalline ferritic iron is reasonably explained by the Hall-Petch model considering dislocation pile-up against grain boundary and dislocation emission from the grain boundary where stress concentration has been generated by piled up dislocations. It is seemed that the segregated carbon stabilized the dislocation emission site at grain boundary and this leads to the increase in ky.