Papers by Author: Goro Miyamoto

Abstract: Acicular ferrite (AFα) formed on oxide particles in steel weld metals has a positive effect on toughness at low temperature. Good lattice coherency between AFα and oxide is one of the proposed reasons for promotion of AFα formation. Lattice coherency is affected by crystal structure of oxide and crystal orientation relationship (OR) between oxide and AFα. In the present study, ORs among AFα, oxide and γFe are investigated in a low carbon steel weld metal. Cube-cube (C-C) OR is observed between γFe and the oxide. It is probable that the oxide liquefied at high temperature, and then crystalized having the C-C OR with the surrounding γFe during cooling in welding process. Near Kurdjumov-Sachs (K-S) and near Baker-Nutting (B-N) ORs are observed between γFe/AFα and oxide/AFα, respectively. The misorientation from the B-N OR is larger than that from the K-S OR just after nucleation of AFα. This implies that AFα forms satisfying a near K-S OR with γFe essentially. It is supposed that formation of both the C-C (γFe/oxide) and near K-S (AFα/γ) ORs results in apparent formation of the near B-N OR between oxide and AFα.

Abstract: The recrystallization behavior of hot-deformed austenite of 0.55% C low alloy steels at 900, 850 and 800°C was investigated by a conventional double-hit compression test and a new method which reconstructs the parent austenite orientation map from an EBSD (electron backscattering diffraction) orientation map of daughter lath martensite. The new method can clearly reconstruct the parent austenite structure at high temperature from the daughter lath martensite structure and we can obtain the information on crystal orientation of the work-hardened austenite. It was revealed that recrystallization of austenite at 800 °C is significantly retarded by the addition of 0.1% V. The strong texture of <110> parallel to the compression direction develops just after the hot-deformation, but this texture becomes weaker as the recrystallization progresses. By applying the reconstruction method, it becomes possible to evaluate various phenomena related to the hot-deformation of austenite

Abstract: The effect of titanium carbide (TiC) on morphology of low-carbon steel martensite was studied by means of electron backscatter diffraction (EBSD). The nucleation and growth of new morphology subunits such as packet, block and sub-block are observed in the area surrounding of micron-sized TiC particles. The misorientation from a fitted orientation relationship between martensite and austenite near TiC particle is larger than the average misorientation with a localized characteristic. The position of new morphology subunits has a well correspondence with the area in vicinity of TiC particle, which has large misorientation. The micron-sized TiC particle plays a role of stress concentrator in austenite during martensitic transformation which suppresses growth of one martensite variant while stimulates nucleation and growth of another one.

Abstract: To Understand the Mechanisms of Accelerated Dynamic Recrystallization Behavior during the Warm Deformation of Martensites, the Tempered Lath Martensite of 0.4C Steel (Fe-0.399%C-1.96%Mn in Mass %) Was Deformed at 650 °C in Compression to Different Reductions, and Microstructural Evolution Was Investigated. During the Deformation, an Initial Lath Martensite Structure with a Complicated Morphology Was Gradually Changed into More Equiaxed Structure. After 50% Reduction and above, an Equiaxed, Fine Grained Structure Mainly Surrounded by High-Angle Boundaries Was Uniformly Formed with Dislocation Substructures, where the Dislocation Density in the Grains Is Relatively Low. Since there Was No Significant Boundary Migration during this Process, this Microstructural Evolution Can Be Termed as Continuous Dynamic Recrystallization.

Abstract: Effects of transformation temperature on variant grouping tendency of bainitic ferrite in a low carbon low alloy steel transformed isothermally are investigated by means of electron backscatter diffraction analysis. Baintic variants of Kurdjumov-Sachs (K-S) orientation relationship belonging to the same Bain correspondence tend to form adjacently in the bainite structure formed at 823K, while the K-S variants sharing the same close-packed plane parallel relation form adjacently in the bainite structure formed at 723K and lath martensite formed by quenching.

Abstract: Demands for medium carbon steels with high strength used for forging parts in automobile have been increasing. V addition to such steels leads to interphase boundary precipitation (IBP) of VC and thus an increase of strength. However, mechanism and strengthening effect of IBP have not been clarified in detail. In this study, precipitation of VC accompanying ferrite and pearlite transformations and its effect on hardness have been examined in medium carbon steels microalloyed with 0.3%V. Specimens transformed in a temperature range between 873 and 973K consist of pearlite and small amount of proeutectoid ferrite. Hardness increase by the V addition becomes larger by lowering transformation temperature at these temperatures. Meanwhile the alloying effect of V on the hardness remarkably decreases at 823K where bainite transformation takes place partly. TEM characterization has revealed that VC are precipitated in both of proeutectoid and pearlitic ferrites in the manner of fine rows parallel to the austenite / ferrite interphase boundary. The size of VC decreases and its number density increases by lowering transformation temperature, corresponding to the larger hardness increase. Orientation relationship analyses between ferrite and austenite in the V-added specimen based of EBSD measurements reveals that proeutectoid ferrite grows preferentially towards an austenite grain with which ferrite does not hold a specific orientation relationship, indicating that classical ledge mechanism does not play a role for interphase boundary precipitation of VC in this alloy.

Abstract: The dislocation density in ferrite and austenite of a bainitic microstructure obtained by transformation at very low temperature (300 °C) has been determined using transmission electron microscopy. Observations revealed that bainitic ferrite plates consist of two distinctive regions with different substructures. A central region in the ferrite plate is observed with dislocations that may result from lattice-invariant deformation at the earlier stage of bainite growth. As plastic deformation occurs in the surrounding austenite to accommodate the transformation strain as growth progresses, the Ferrite/Austenite interface has also a very distinctive dislocation profile. In addition, atom-probe tomography suggested that dislocation tangles observed in the vicinity of the ferrite/austenite interface might trap higher amount of carbon than single dislocations inside the bainitic ferrite plate.

Abstract: Orientation relationships of proeutectoid ferrite formed at an austenite grain boundary with respect to adjacent austenite grains were investigated by means of electron backscatter diffraction in an isothermally transformed Fe-1.5Mn-0.2C (mass%) alloy. A grain bounadry ferrite holds nearly the Kurdjumov-Sachs (K-S) orientation relationship with some small misorientation against at least one of the adjacent austenite grains. There is strong variant selection by the austenite grain boundary for ferrite nucleation. At a higher transformation temperature, the fraction of ferrites holding a near K-S relationship with respect to the opposite austenite grain is lower. As the transformation temperature becomes lower, the misorientation from the K-S relationship becomes smaller against the near K-S related austenite grain whereas the misorientation against the irrationally oriented austenite grain becomes larger.

Abstract: Reverse transformation has been frequently used to refine austenite grain size for refining ferrite, pearlite and martensite structures. However, kinetics and microstructure change during reverse transformation to austenite has not been examined systematically compared with the austenite decomposition reaction. Therefore, alloying effects of 1mass% Mn, Si and Cr on reverse transformation kinetics from pearlite and tempered martensite structures in Fe-0.6mass%C alloys were investigated in this study. Vickers hardness of all the specimens increases with increasing holding time at 1073K because reversely-formed austenite transforms to martensite by quenching. In the reverse transformation from pearlite structure, the kinetics of reverse transformation is hardly changed by the Mn addition while Si and Cr additions delay it. Kinetics of reverse transformation from tempered martensite structure becomes slower than from the pearlite structure in all the alloys. In particular, retarding effect by the Cr addition is most significant among those elements.

Abstract: Fe-0.15%C-1.5%Mn-0.2%Si (Nb-free alloy) and Fe-0.15%C-1.5%Mn-0.2%Si-0.03%Nb (Nb-added alloy) were continuously cooled to room temperature at constant cooling rates in the range from 0.1 to 20K/s. At lower cooling rates, such as 0.1K/s, the Nb addition retards the ferrite transformation, resulting in a decrease in the transformation temperature and an increase in the volume fraction of bainite. The fraction of martensite-austenite constituent (MA) increases by the Nb addition and the largest fraction of MA, about 0.5 %, is observed in the Nb-added specimen cooled at 5K/s. In the specimens cooled at 5K/s, relatively coarse bainite without cementite precipitation is formed near the austenite () grain boundary in both alloys. Most of MA is localized between such relatively coarse bainitic ferrite (BF). On the other hand, MA is hardly observed in the bainite formed with cementite precipitation in  grain. Based on microstructure observation of the continuously cooled specimens down to intermediate temperatures followed by quenching, it is concluded that small-sized untransformed  near  grain boundary partly remains as MA whereas relatively larger untransformed  in the  grain decompose into bainite with cementite precipitation.