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1) Graduate student, Faculty of Engineering, Ibaraki University, Hitachi, 316-8511 Japan. 2) Graduate student, on leave from International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan, 430081 China. 3) Department of Materials Science and Engineering, Ibaraki University, Hitachi, 316-8511 Japan. 4) Department of Mechanical Engineering, Graduate School of Science and Engineering, Kagoshima University, Kagoshima, 890-8580 Japan.

In this study, three different microstructures of dual-phase (DP) steel obtained by varying the intercritical annealing temperature were observed three dimensionally by a serial-sectioning method and their metrics and topological properties were quantified. The results demonstrated that martensite in DP steels has various morphologies such as layers, particles, holes, and voids. The fraction of these characteristic morphologies was examined as a function of the intercritical annealing temperature.

1) National Institute for Materials Science, Innovative Materials Engineering Laboratory, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 Japan. 2) Foremerly at National Institute for Materials Science, Innovative Materials Engineering Laboratory. Nowat Department of Mechanical Engineering, Graduate School of Science and Engineering, Kagoshima University, Korimoto 1-21-24, Kagoshima, 890-8580 Japan. E-mail: adachi@mech.kagoshima-u.ac.jp(

Physical meaning of dynamic transformation was re-considered with a particular attention to the effect of post-transformation deformation on crystallography and kinetics of transformed ferrite. An advanced in-situ neutron diffraction experiment was performed to examine the microstructural evolution besides an advanced EBSD measurement. In particular, the deformation behavior in austenite/ferrite two-phase region was investigated as a function of deformation temperature and ferrite volume fraction. Based on these findings, the specific features of dynamic transformation were extensively discussed.

An Fe-3%Si bicrystal was deformed in compression to a strain of 0.9 at ambient temperature. In the interior of deformation bands, characteristic band structures with high orientation gradients and low and high angle boundaries were formed during straining. Furthermore, isolated ultrafine grains were found in the matrix of the deformation bands. The morphology and crystallography of the ultrafine grains indicate that they are caused by slip system partitioning, i.e. local activity of a highly stressed slip system that is different from the active slip system in the surrounding crystal volume.

A novel combined technique of neutron diffraction and electron back scattering diffraction was applied to examine hierarchical deformation behavior of 18 mass% Ni martensitic steel. In-situ neutron diffraction experiment during tensile deformation demonstrated that intergranular stress was generated. EBSD analysis suggested that slip bands terminated not only at block boundaries, but also sub-block boundaries at a relatively small strain. In many cases, slip bands crossing sub-block boundaries were zigzagged. With increasing strain, sub-block became unclear and then block boundaries worked as a main barrier for dislocation gliding. Such kind of heterogeneous plastic flow in differently oriented hkl blocks seems to be a possible reason for the intergranular stresses.

The microstructures of two high-nickel martensite steels with different carbon contents before and after warm compression were comparably investigated by using a field-emission scanning electron microscope attached with an electron backscattering diffraction equipment (FE-SEM/EBSD) and a transmission electron microscope attached with an energy dispersive X-ray spectroscope (TEM/EDX). The microstructure observations suggest that the carbon addition is beneficial to reduce the critical strain for full recrystallization during warm compression and the increment of carbon-enriched austenite amount accelerates the dynamic recrystallization of ferrite through plastic deformation partitioning in the 17Ni–0.2C martensite steel. Proper pre-tempering promotes the precipitation of the carbon- and nickel-enriched austenite, and then promotes the dynamic recrystallization. On the other hand, long-time tempering leads to the carbon depletion in austenite so as to delay the dynamic recrystallization. The full recrystallization of the 18Ni martensite steel takes place at a higher strain during warm compression, mainly because of no carbon-enriched austenite.

The dynamic recrystallization behavior during warm compression for a low carbon martensite steel was investigated to make clear the effects of initial martensite block size, compression strain and pre-tempering before compression. It is found that the average size of recrystallized ferrite grains is influenced neither by the initial martensite block size (austenitizing temperature) nor by the amount of compression strain. The pre-tempering before compression shows two competitive effects: cementite particles precipitated during pre-tempering at 600°C promotes the occurrence of dynamic recrystallization while the decrease in dislocation density during pre-tempering at a higher temperature delays the dynamic recrystallization. Dispersed cementite particles suppress ferrite grain growth. Hence, there is an optimum tempering condition before warm-compression in order to obtain fine grained microstructure.

Effects of light deformations before and after precipitation (pre-deformation and post-deformation) on orientation distribution of intergranular bcc-precipitates were examined in a Ni-43mass%Cr alloy by electron backscattered diffraction. It was found that the deviation angles from both the plane/direction parallel orientation relationships in the Kurdjumov-Sachs orientation relationship of intergranular precipitates were increased by both pre- and post-deformations, but post-deformation was more effective. Based on this result, microstructural evolution of intergranular precipitates formed dynamically was discussed with particular attention to coherent-to-incoherent transition of precipitates.

The effect of the microstructure of the base steel on the formation and growth behavior of intermetallics on interstitial free steel during galvanizing under various processing conditions (substrate entry temperature, Al content in galvanizing bath) was investigated from the viewpoint of the surface morphology of the intermetallics. In addition, the surface morphology of galvannealed coating was also examined. The dependency of Fe-Zn intermetallics growth behavior on the crystal orientation of the base steel was observed clearly in galvanizing in 0.10 mass% or less Al bath, especially at low entry temperatures. In general, ζ crystals are precipitated orderly on (111)α, whereas those are precipitated disorderly on (001)α and (101)α at the outset of the formation of Fe-Zn intermetallics, and the growth rate of Fe-Zn intermetallics on (001)α and (101)α is larger than that on (111)α. As an exception, the formation of Fe-Zn intermetallics on (001)α and (101)α at the outset of Fe-Zn interaction is suppressed at the entry temperature of 673K in 0.10 mass%. Al bath, although the clump of orderly precipitated ζ is formed on (111)α. The growth of Fe-Zn intermetallics during galvannealing also depends on crystallographic orientation of the αFe grain of the base steel, being retarded on (111)α. A large αFe grain with (111)α on the surface is thought to cause a concave portion on the galvannealed surface. Consequently, it is thought that the formation and growth behavior of Fe-Zn intermetallics on IF steel is affected not only by the αFe grain boundary but also by the crystallographic orientation of the αFe grain of the base steel.

The effect of Fe content of coating and Al content in galvanizing bath on the coating adhesion of galvannealed ultra-low carbon IF (Interstitial Free) steel sheet was investigated using a lap-shear method. The exposed steel surface after removal of the galvannealed coating with hydrochloric acid solution was examined so as to discuss the relation between the geometrical shape of the coating/steel interface and the coating adhesion.Galvannealed coating exhibits the minimum adhesion strength in the region of 8-10 mass% Fe, which was thought to relate to the extinction of η-phase. Further increment of Fe content raised the coating adhesion, and high Al addition to galvanizing bath also raised the minimum adhesion strength of the coating around 9 mass%Fe.The steel surface exposed with the acid was even and smooth for the low Al content coatings with 8-9 mass% Fe, while that was uneven or rugged for the coatings with relatively high coating adhesion. The further increment of Fe content in the coating developed Γ-phase to form ledges with the pitch of about 100 nm at the coating/steel interface, and the increase of Al in galvanizing bath resulted in uneven alloy formation, particularly discontinuous formation of Γ-phase to cause uneven erosion of the steel surface and form the ruggedness with the pitch comparable with α-Fe grain size at the coating/steel interface. The ruggedness with the pitch comparable with α-Fe grain size was thought to result from the dependency of Fe-Zn interaction on the microstructure of the base steel. It was assumed that the coating adhesion is governed by the ruggedness of the coating/steel interface and formation of Γ-phase contributes to increasing the adhesion strength.