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This study is concerned with the effects of microstructure on Charpy V-notch impact properties of high-toughness API X70 and X80 line-pipe steels. Three kinds of steels were fabricated by varying alloying elements and hot-rolling conditions, and their microstructures and Charpy impact properties were investigated. In addition, their effective grain sizes were characterized by the electron back-scatter diffraction (EBSD) analysis. The Charpy impact test results indicated that the steels rolled in the single phase region had the higher upper shelf energy (USE) than the steel rolled in the two phase region because their microstructures were composed of acicular ferrites. In the X80 steel rolled in the single phase region, the decreased energy transition temperature (ETT) could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. Thus, it had excellent mechanical properties including high yield and tensile strengths, high USE, and low ETT, although its elongation was low.

keyword : API X70 and X80 Line-pipe steel, Charpy impact properties, Energy transition temperature, Effective grain size

In order to clarify the deformation behavior under the conditions of multiple slip activations, the plane strain compression tests were carried out on single crystals of Ni/Ni3Al two-phase (Ni-18 at.%Al) alloys. The flow behaviors were strongly dependent on the initial crystal orientations and were found to be divided into two types depending on the operative number of slip planes. When slip (s) on 1 or 2 slip planes were activated, the work-hardening rates decreased before the critical shear stress levels which was in marked contrast to the case of single phase Ni3Al. On the other hand, the work-hardening rates kept high level in the case of the multiple slips activations on 3 or 4 slip planes. Such flow behaviors were considered to be closely related to differences in deformation between Ni matrix and Ni3Al precipitates.

keyword : Ni/Ni3Al single crystals, Plane strain compression, Taylor-Bishop-Hill analysis

2024 Al alloy was severely deformed by equal channel angular pressing (ECAP) to obtain an ultra-fine grain structure. The relationship between the grain size and deformation amount was investigated by high resolution TEM. The shape of S` phase was observed to identify the effect of severe plastic deformation. As the deformation amount increased, the grain size decreased, but after 1 pass, the decrease of grain size was small. The grain structure was changed from irregular into equiaxed. After 6 pass, the grain size of the specimen was about 100 nm, and the size of S` phases was about 10 nm. The morphology of S` phases was changed to spherical from rod-type by severe plastic deformation.

keyword : ECAP, 2024 Al alloy, S` phase, Microstructure, Subgrain

Deformation behavior of nanocrystalline (nc) aluminum ana aluminum alloys, Al93(Fe2Cr1.5Ti1.5) Mn2 and Al93(Fe2Cr1.5Ti1.5)Si2, has been investigated. The specimens are fabricated by hot-extrusion of the nc powders produced by mechanical ball milling. X-ray diffraction and transmission electron microscopy analyses reveal that grain sizes of aluminum and aluminum alloys are around 50 nm and 56 nm, respectively. The nc aluminum shows superior strength around 460 MPa, and Al93(Fe2Cr1.5Ti1.5)Mn2 alloy containing nano-scale particles exhibits 2 times higher strength than that of nc aluminum. Since there is no ability of accumulating dislocations within the nano-scale grains during deformation, the nc aluminum shows negligible work hardening capacity.

keyword : Nanocrystalline metal, Aluminum, Mechanical properties, Deformation behavior

Hardness and corrosion resistance of surface composites fabricated by high-energy electron-beam irradiation of Fe-based amorphous alloy powders were investigated. Amorphous powders were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these powders without flux to fabricate a one-layered surface composite. A two-layered surface composite was also fabricated by irradiating electron beam again onto the powders deposited on the one-layered surface composite. The two-layered composite contained a large amount (up to 48 vol.%) of hard Cr2B crystalline phases in the Cr0.19Fe0.7Ni0.11 matrix, and thus its hardness was 3 times greater than that of the steel substrate. The corrosion resistance of the two-layered composite was better than that of an stainless steel or a coating fabricated by high-velocity oxygen fuel spraying of Fe-based amorphous powders because it contained Cr2B phases having good corrosion resistance without pores or cracks. These findings suggested that various applications of the fabricated surface composites to materials requiring high resistance to wear and corrosion were expected.

keyword : High-energy electron beam irradiation, Surface composite, Amorphous alloy powder, Corrosion resistance

General methods of evaluating indentation tensile properties using instrumented indentation techniques are aimed at power-law hardening materials. However, some FCC materials with low stacking-fault energy, such as austenitic stainless steels, show linear hardening behavior. In this study, in order to reflect linear hardening behavior, a sine function was adapted in the definition of true strain instead of tangent function. The true strain of sine function was proved to be more suitable to describe linear and high hardening deformation pattern due to its own mathematical features. Accordingly, accurate indentation flow curves were successfully derived with the new true strain definition and a linear constitutive equation. The indentation yield strength and indentation tensile strength were determined as the corresponding true stresses to the yield point and the necking point on the indentation flow curve, respectively. It was found that the indentation yield strengths were within a 10% error range, and the indentation tensile strengths were within a 5% error range, when compared to tensile test results.

keyword : Linear hardening, Instrumented indentation test, Indentation tensile property, True strain

The solidification grain structure is one of the important features that have to be controlled in many casting processes. In order to predict the solidification grain structure in the Ni-base superalloy CM247LC, a three-dimensional cellular automaton (CA) model has been coupled with finite element (FE) heat flow calculations. In the CA-FE model, Gaussian distribution of nucleation sites was assumed to account for the heterogeneous nucleation in the bulk liquid and at the mold wall. The Kurz-Giovanola-Trivedi (KGT) model was extended to multi-component Ni-base superalloy to take account of the growth kinetics of the dendrite tip. For describing Gaussian distribution of nucleation sites at the mold wall, an empirical relationship between the surface grain density and the initial cooling time of the melt was proposed and applied to the input parameters in the present CA-FE model. The simulated grain structures were validated at various casting conditions by comparison with the corresponding experimental micrographs. The CA-FE predictions of grain structure are in good agreement with the experimental results.

keyword : Superalloy, CA-FE model, Grain structure prediction, Surface grain size

The Steel flow pattern in the mold is investigated in a wide slab caster with the bifurcated nozzle of downward 25 degrees that is equipped with Electro Magnetic Level Accelerator (ELMA) system. It applies a low frequency alternating magnetic field with the moving direction from the center of the mold to the narrow face. After solving the fluid dynamics numerically, inclusion particle trajectories are calculated using Lagrangian particle tracking method. The simulation results show that EMLA mode eliminates the negative impact of wide slabs by transforming the single roll pattern generated by argon into optimized double roll pattern. This paper presents the optimum electric current and demonstrates the relevance of flow control and inclusion behaviour.

keyword : Continuous casting, Electromagnetic Level accelerator, Discrete phase model, Single roll, Double roll, Inclusion flotation

A method for measuring the surface heat transfer coefficient, h, is presented, which combines a parameter estimation technique with the fast-response capabilities of thin film thermocouples. The developed method was applied to measure the surface heat transfer coefficient of the STS 304L specimens during quenching from pre-selected temperatures, 300 and 400℃, into RT deionized water bath. The surface of specimens were abraded with #200, #800 abrasive papers and polished with 0.3 micrometer alumina powders to investigate the surface roughness effects on h. The measured h values showed the range 10^(4)~10^(5) w/㎡C depending on the surface roughness, which were related to the mechanisms operating during water boiling.

keyword : Surface heat transfer coefficient, Surface roughness, STS 304L, Parameter estimation method

Three kinds of calcium ferrites, predominant phases in iron ore sinter, were produced by the sintering of a mixture of iron, calcium, silicon and aluminum oxides to understand the phases formation process and to evaluate the properties of sinter phases. The synthesized binary, ternary and quaternary calcium ferrite phases were analyzed by microscopic, X-ray diffraction and energy dispersive analysis method and reduced at 550℃ with CO gas for reduction degradation strength test. The addition of Al2O3 to Fe2O3CaO or Fe2O3-CaO-SiO2 system encouraged the formation of calcium ferrite and prevented hematite. The same effects were obtained by adding CaO, whereas Si02 promoted the hematite formation when insufficient of CaO provided to the system. Adding SiO2 to the system more than 6%, Fe2O3 did not participate to form calcium ferrites and precipitated as skeletal rhombohedral hematite phase which has poor reduction degradation strength. As the CaO content in calcium ferrites increased, reduction degradation strength of sinter was improved.

keyword : Iron ore sinter, Calcium ferrites, Reduction degradation strength, Al2O3, Hematite