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Kotobu Nagai - One of the best experts on this subject based on the ideXlab platform.

  • Effect of initial Grain Size on evolved Ferrite Grain Size during high Z large strain deformation
    Materials Science and Technology, 2010
    Co-Authors: S.v.s. Narayana Murty, Kotobu Nagai, Shiro Torizuka, Takayoshi Kitai, Yasuo Kogo
    Abstract:

    AbstractThe effect of initial Grain Size on the evolved Ferrite Grain Size during high Z large strain deformation was studied in ultralow carbon steel with two widely varying initial microstructures: a coarse Grained (200 μm) and an ultrafine Grained (0·7 μm) microstructures. Plane strain compression was used for imposing large strain in the test specimen and a thermomechanical simulator was used for varying the strain rate and temperature precisely. Electron backscattered diffraction was used for the analysis of deformed microstructure and for Grain Size measurements. Deformation conditions were varied to cover wide range of Zener–Hollomon parameters. It was noted that the evolved Ferrite Grain Size solely depends on the Zener–Hollomon parameter and does not depend on the initial Grain Size. The relation d=10Z–0·12 (Q gb=155 kJ mol–1) was obtained for two initial microstructures with Grain Sizes varying by three orders of magnitude.

  • Effect of Ferrite Grain Size on tensile deformation behavior of a Ferrite-cementite low carbon steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
    Co-Authors: Noriyuki Tsuchida, Yo Tomota, Yasunori Harada, Kenzo Fukaura, H. Masuda, Kotobu Nagai
    Abstract:

    Abstract Stress–strain curves for Ferrite-cementite (FC) steels with Ferrite Grain Sizes between 0.47 and 13.6 μm were studied by tensile tests with strain rates of 103, 100, and 3.3 × 10−4 s−1 at 296 K. The stress–strain curves for the FC steels are categorized into two different types. In one type, the Luders deformation is interrupted due to the onset of necking, and in the other type, the Luders band propagates throughout the gage section of a tensile specimen followed by work-hardening. The lower yield and flow stresses increase while uniform and total elongations decrease with a decrease in Ferrite Grain Size. The effect of Ferrite Grain Size on flow stress is hardly dependent on strain rate. These experimental results reveal that the Grain refinement strengthening contributes mainly to an increase in the athermal stress component.

  • Flow Stress Analysis using the Kocks–Mecking Model for Ferrite–Cementite Steels with Various Ferrite Grain Sizes
    Isij International, 2008
    Co-Authors: Noriyuki Tsuchida, Kenzo Fukaura, Kotobu Nagai, Yo Tomota
    Abstract:

    True stress (σ)–true strain (e) curves were calculated by using the Kocks–Mecking (KM) model for the Ferrite–cementite steels with various Ferrite Grain Sizes between 0.47 and 13.6 μm. In the KM model, the effect of Ferrite Grain Size on flow stress is described by the athermal stress component that follows the Hall–Petch equation. The effects of temperature and strain rate on flow stress, which are correlated with the thermal stress component, are independent of the Ferrite Grain Size. The calculated σ–e curves by using the KM model agree with the measured ones at various temperatures and strain rates including the high-speed tensile test with a strain rate of 103 s−1. From the calculations based on a micromechanic model, it is found that the volume fraction of second phase affects the Grain Size dependence in multi-phase steels. The m-value showing strain rate sensitivity for the external stress was decreased with a decrease in Grain Size and that for the thermal stress was independent of Grain Size.

  • relationship between yield strength and Grain Size for a bimodal structural ultrafine Grained Ferrite cementite steel
    Scripta Materialia, 2007
    Co-Authors: Kotobu Nagai, Toshihiro Hanamura, Ming-chun Zhao, Fuxing Yin, Andrej Atrens
    Abstract:

    A bimodal Ferrite Grain Size distribution was developed in the microstructure of a warm-rolled ultrafine-Grained Ferrite/cementite steel by annealing. The yield strength could be illustrated by the Hall-Petch relation, where the appropriate Grain Size was the average Ferrite Grain Size determined from the area fraction of the inverse of the average Grain Size in both the large- and small-Sized regions. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • Relationship between yield strength and Grain Size for a bimodal structural ultrafine-Grained Ferrite/cementite steel
    Scripta Materialia, 2007
    Co-Authors: Ming-chun Zhao, Kotobu Nagai, Toshihiro Hanamura, Fuxing Yin, Andrej Atrens
    Abstract:

    A bimodal Ferrite Grain Size distribution was developed in the microstructure of a warm-rolled ultrafine-Grained Ferrite/cementite steel by annealing. The yield strength could be illustrated by the Hall-Petch relation, where the appropriate Grain Size was the average Ferrite Grain Size determined from the area fraction of the inverse of the average Grain Size in both the large- and small-Sized regions. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Yo Tomota - One of the best experts on this subject based on the ideXlab platform.

  • Effect of Ferrite Grain Size on tensile deformation behavior of a Ferrite-cementite low carbon steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
    Co-Authors: Noriyuki Tsuchida, Yo Tomota, Yasunori Harada, Kenzo Fukaura, H. Masuda, Kotobu Nagai
    Abstract:

    Abstract Stress–strain curves for Ferrite-cementite (FC) steels with Ferrite Grain Sizes between 0.47 and 13.6 μm were studied by tensile tests with strain rates of 103, 100, and 3.3 × 10−4 s−1 at 296 K. The stress–strain curves for the FC steels are categorized into two different types. In one type, the Luders deformation is interrupted due to the onset of necking, and in the other type, the Luders band propagates throughout the gage section of a tensile specimen followed by work-hardening. The lower yield and flow stresses increase while uniform and total elongations decrease with a decrease in Ferrite Grain Size. The effect of Ferrite Grain Size on flow stress is hardly dependent on strain rate. These experimental results reveal that the Grain refinement strengthening contributes mainly to an increase in the athermal stress component.

  • Flow Stress Analysis using the Kocks–Mecking Model for Ferrite–Cementite Steels with Various Ferrite Grain Sizes
    Isij International, 2008
    Co-Authors: Noriyuki Tsuchida, Kenzo Fukaura, Kotobu Nagai, Yo Tomota
    Abstract:

    True stress (σ)–true strain (e) curves were calculated by using the Kocks–Mecking (KM) model for the Ferrite–cementite steels with various Ferrite Grain Sizes between 0.47 and 13.6 μm. In the KM model, the effect of Ferrite Grain Size on flow stress is described by the athermal stress component that follows the Hall–Petch equation. The effects of temperature and strain rate on flow stress, which are correlated with the thermal stress component, are independent of the Ferrite Grain Size. The calculated σ–e curves by using the KM model agree with the measured ones at various temperatures and strain rates including the high-speed tensile test with a strain rate of 103 s−1. From the calculations based on a micromechanic model, it is found that the volume fraction of second phase affects the Grain Size dependence in multi-phase steels. The m-value showing strain rate sensitivity for the external stress was decreased with a decrease in Grain Size and that for the thermal stress was independent of Grain Size.

  • tensile behavior of trip aided multi phase steels studied by in situ neutron diffraction
    Acta Materialia, 2004
    Co-Authors: Yo Tomota, H Tokuda, Yoshitaka Adachi, Masayuki Wakita, Nobuaki Minakawa, A Moriai, Yukio Morii
    Abstract:

    Abstract TRIP-aided multi-phase steels were made by thermo-mechanically controlled process, where the Ferrite Grain Size and the amount of the retained austenite were changed by controlling process conditions. The tensile behavior of four steels was studied by in situ neutron diffraction. It is found that the retained austenite bearing about 1.0 wt% C is plastically harder than the Ferrite matrix. The steel with a Ferrite Grain Size of ≈2.0 μm showed tensile strength of 1.1 GPa and a uniform elongation of 18.4%, in which stress-induced martensitic transformation occurs during plastic deformation but a considerable amount of austenite remains even after the onset of necking. It is concluded that the enhancement of uniform elongation is caused mainly by the work-hardening due to the hard austenite and martensite, where the contribution of the transformation strain is negligible.

  • In situ neutron diffraction during tensile straining of fine Grained Ferrite - pearlite steel
    Materials Science and Technology, 2004
    Co-Authors: Takumi Ono, Noriyuki Tsuchida, Yo Tomota, P. Lukas, D. Lugovy, D. Neov, Kotobu Nagai
    Abstract:

    Abstract In situ neutron diffraction during tensile deformation was performed on a steel with Ferrite Grain Size 3.6 μm, which was close to the achievable minimum Size for a Ferrite - pearlite structure. The lattice plane strain, microstrain related to dislocation density, and the Size of coherently diffracting blocks related to dislocation cell Size were determined from the profile analysis of (110) diffraction spectra, as a function of the applied stress. The change in lattice plane strain revealed that Grain refining strongly influences 'Grain to Grain yielding' behaviour with respect to Luders deformation. The increase in dislocation density estimated in the uniformly extending regime was similar to that observed in another specimen with Ferrite Grain Size 46.2 μm.

Peter Hodgson - One of the best experts on this subject based on the ideXlab platform.

  • Effect of transformation mechanism (static or dynamic) on final Ferrite Grain Size
    Materials Science and Technology, 2009
    Co-Authors: Alireza Shokouhi, Peter Hodgson
    Abstract:

    A simple series of test was developed to highlight and compare the difference between the static strain induced transformation (SSIT) and the dynamic strain induced transformation (DSIT) mechanism in Grain refinement and also to investigate the origin of the difference between the two mechanisms. The results showed that while the SSIT sets up a two-dimensional impingement among the Ferrite Grains, it cannot avoid their coarsening (normal growth). However, the DSIT forms a group of Grains with a three-dimensional impingement which does not coarsen and maintains their fine Size throughout the transformation, thereby, reduces the final average Grain Size.

  • Ultrafine Grained structure formation in steels using dynamic strain induced transformation processing
    International Materials Reviews, 2007
    Co-Authors: Hossein Beladi, Georgina Kelly, Peter Hodgson
    Abstract:

    The refinement of Ferrite Grain Size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of Ferrite refinement is limited...

  • analysis and characterisation of ultra fine Ferrite produced during a new steel strip rolling process
    Scripta Materialia, 1999
    Co-Authors: P J Hurley, Peter Hodgson, Barry C Muddle
    Abstract:

    The designing of processing routes that minimize the final Ferrite Grain Size is essential for the development of high strength steels with improved toughness and ductility. In this paper, a novel procedure for producing ultra-fine Ferrite is investigated. This method is attractive in terms of its relative simplicity and ability to refine the Ferrite Grain Size in relatively low cost steels. In an earlier paper, it was stated that the high level of Ferrite Grain refinement occurring during this process was likely to be the result of a strain-induced transformation mechanism. Thus, it has been termed the SITR (strain-induced transformation rolling) process. In the present paper, detailed characterization of the fine Ferrite produced using this technique has been performed with the aim of providing a deeper insight into the important factors giving rise to its generation.

  • Ferrite Grain coarsening during transformation of thermomechanically processed C–Mn–Nb austenite
    Materials Science and Technology, 1992
    Co-Authors: R. Priestner, Peter Hodgson
    Abstract:

    AbstractThe Ferrite Grain Size of low carbon steel is known to be refined by hot rolling in the austenite phase field at temperatures too low for recrystallisation to occur. The strain thus retained in the austenite increases Ferrite nucleation density and in current models of Grain refinement it is assumed that each nucleus becomes a Grain in the fully transformed microstructure. In this paper it is shown that, in a heavily deformed C–Mn–Nb austenite, Ferrite Grains impinged, then coarsened during the initial stages of transformation during continuous cooling. The final Ferrite Grain Size was not established until 35% of transformation had occurred. It is suggested, firstly, that Ferrite Grain refinement due to controlled rolling cannot be modelled simply from observed increases in nucleation density and, secondly, that deformation of austenite has considerably greater potential for Grain refinement than is commonly observed, provided that coarsening of the Ferrite during transformation can be limited.MS...

Yukio Morii - One of the best experts on this subject based on the ideXlab platform.

  • tensile behavior of trip aided multi phase steels studied by in situ neutron diffraction
    Acta Materialia, 2004
    Co-Authors: Yo Tomota, H Tokuda, Yoshitaka Adachi, Masayuki Wakita, Nobuaki Minakawa, A Moriai, Yukio Morii
    Abstract:

    Abstract TRIP-aided multi-phase steels were made by thermo-mechanically controlled process, where the Ferrite Grain Size and the amount of the retained austenite were changed by controlling process conditions. The tensile behavior of four steels was studied by in situ neutron diffraction. It is found that the retained austenite bearing about 1.0 wt% C is plastically harder than the Ferrite matrix. The steel with a Ferrite Grain Size of ≈2.0 μm showed tensile strength of 1.1 GPa and a uniform elongation of 18.4%, in which stress-induced martensitic transformation occurs during plastic deformation but a considerable amount of austenite remains even after the onset of necking. It is concluded that the enhancement of uniform elongation is caused mainly by the work-hardening due to the hard austenite and martensite, where the contribution of the transformation strain is negligible.

Barry C Muddle - One of the best experts on this subject based on the ideXlab platform.

  • analysis and characterisation of ultra fine Ferrite produced during a new steel strip rolling process
    Scripta Materialia, 1999
    Co-Authors: P J Hurley, Peter Hodgson, Barry C Muddle
    Abstract:

    The designing of processing routes that minimize the final Ferrite Grain Size is essential for the development of high strength steels with improved toughness and ductility. In this paper, a novel procedure for producing ultra-fine Ferrite is investigated. This method is attractive in terms of its relative simplicity and ability to refine the Ferrite Grain Size in relatively low cost steels. In an earlier paper, it was stated that the high level of Ferrite Grain refinement occurring during this process was likely to be the result of a strain-induced transformation mechanism. Thus, it has been termed the SITR (strain-induced transformation rolling) process. In the present paper, detailed characterization of the fine Ferrite produced using this technique has been performed with the aim of providing a deeper insight into the important factors giving rise to its generation.