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High boron containing Fe-Cr alloy powder has been investigated in this research as to its microstructure evolution by the rapid solidification in thermal spraying process. The Fe-Cr alloy powder consists of Cr-rich metal borides, such as Cr_(1.65)Fe_(0.35)B_(0.96) and Cr₂B, within Fe-Cr solid solution matrix phase. In the alloy powder, the boron content in the matrix phase was extremely low. Boron was supplied to the matrix Fe-Cr solid solution phase by the thermal dissolution of Cr-rich boride particles during the thermal spraying. Cross-sectional TEM observation on the coatings indicates that the coated layer is composed of equiaxed nanocrystalline Fe-Cr solid solution phase and boride particles. The formation of equiaxed nanocrystalline phase indicates that large undercooling could be attained during the rapid solidification of thermal spraying process.

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The process of triangular prism formation and abnormal grain growth of WC was modeled using pseudo Monte-Carlo simulation based on atomic adsorption and coalescence mechanism. Grains of WC evolved into the triangular prism shape due to {101￣0} and {12￣10} planes of fast growth rate and subsequent coalescence of {101￣0} and {11￣00} planes were the main reason for the abnormal grain growth. The probability of coalescence computed by the Monte-Carlo method agreed well with the theoretical value. Experimental evaluation of the computational model was made in sintered WC-25wt.%Co alloy. The experimental alloy was made with WC powder of different particle size, 0.8㎛ and 325mesh, respectively, and with two different sintering conditions: solid phase sintering at 1250℃ for 12h and liquid phase sintering at 1370℃ for 72h. The alloy made with the fine powder assumed the triangular prism shape quickly during solid phase sintering, and their growth characteristics could be explained by the atomic adsorption and coalescence models.

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The shape memory effects of CuZnAl shape memory alloys are based on martensitic phase transformation from a high-symmetrical austenite (cubic crystal symmetry) to low-symmetrical martensite (orthorhombic or monoclinic crystal symmetry) and back, whereby a distinctive crystallographic orientation relationship between austenite and martensite exist. If this phase transformation takes place in a polycrystalline material with a particular texture, then the martensitic phase inherits a texture from the starting austenite texture. Therefore the crystallographic orientation relationship between both phases can be determined by the austenite texture and the martensite texture. The austenite texture of a extruded and hot-rolled Cu-14.2wt.%Zn-8.5wt.%Al-0.5wt.%Ti sheet was measured at 115℃ and the martensite texture at RT. With the measured texture of the martensitic and austenitic state the crystallographic orientation relationship was determined. The pseudoelastic property (shape memory effect) and the Young`s modulus were calculated with the measured austenite texture and with the calculated austenitic texture from the martensite texture using the texture transformation.

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The effects of distribution of deoxidized product by Mg and dissolved Mg content on abnormal grain growth had been carried out in Fe-0.2wt%C-0.02wt%P alloys maintained at 1673 K for 60 minutes. Homogeneous distribution of deoxidized product during solidification was strongly depended on the cooling rate, and degree of that was conformed by homogeneous distribution degree(H.D.D.) defined by author. In the cooling rate of 40 K/min, the distribution of deoxidized product was almost homogeneous. A_(max)/A_(mean), which is widely used to represent the degree of abnormality, was depended more on the content of dissolved Mg than on the volume fraction of deoxidized product in the condition of 40 K/min cooling rate. And. A_(max)/A_(mean) was also deeply related with the aspect ratio (the ratio of the longest length of grain to the longest width perpendicular to the longest length)

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This study was performed to clarify the effect of temper rolling conditions on the formation of abnormally coarse grain(ACG) structure in extra-low carbon steel. Both laboratory and mill trial temper rollings were carried out with various rolling reductions, followed by annealing at temperatures ranging between 660℃ and 700℃ for 3 hours. Optical microstructure and texture were examined using annealed specimens. FEM analysis was also performed to compare the effects of roll diameter, friction, and tension. The ACG was observed when the rolling reduction was higher than a critical value, which was found to be decreased at higher annealing temperatures. The formation of the coarse grains started at the surface layer of the specimens. Crystal orientation was not changed with the development of ACG, but the intensity of texture was decreased. When the rolling reduction was 2%, the ACG could be obtained in the specimens rolled in the mill. However, the ACG was not observed in the laboratory. FEM analyses showed that the discrepency between the two rolling mills was caused by the differences in the roll diameter and the friction coefficient.

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In 21st century, a lot of changes is expected in industrial and social circumstance due to the fast development in technology and high performance structural steels are required to response to the new environment more efficiently. Remakable improvements in strength, toughness and weldability and good recyclability are discussed as the quality requirements for new high performance structural steels. Among various metallurgical methods, grain refinement to 1㎛ size was selected as the most promising method to meet the quality requirement of new structural steels. Multi-variant, multi-axis deformation, deformation induced transformation and the use of high magnetic field have been discussed as the new technologies for ultra grain refining. In this paper, theoretical background and recent experimental results of new technologies are critically reviewed and the future research and developing direction of ultra fine grain steel was discussed.

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The development of texture during hot rolling of Ti/Nb-bearing interstitial free steel was investigated for various processing parameters. The texture of the resulting ferrite after hot rolling is related to that of the parent austenite. The transformation from the austenite texture to the ferrite is suggested to be {112}_γ {225}_γ//{001}_α and <111>_γ <554>_γ//<110>_α where γ and α stand for austenite and ferrite, respectively, and are interchangeable.

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As a preliminary study of developing 800MPa tensile strength grade structural steel plates, the ferrite grain refinement was investigated in a plain low carbon steel. Steels with the composition of 0.15%C-1.1%Mn-0.25%Si were heavily deformed in the uniaxial compression mode and cooled down by using a Gleeble 1500 hot deformation simulator. When a specimen was deformed by 80% at a strain rate of 10/s at 775℃ which was higher than Ar3 temperature of the undeformed specimen by 10℃ and then cooled to room temperature at 10℃/sec, a fine microstructure comprising small ferrite grains of about 2㎛ in size and cementites precipitating at grain boundaries was developed. This compressed specimen showed a high Vickers hardness of 210 which could be converted to tensile strength of 720MPa. Ferrite grains exhibited a random crystallographic orientation and misorientation angles between two neighboring grains were mostly larger than 15 degrees. Some grains divided by subgrain boundaries were also observed. From various experimental results, it could be concluded that these fine ferrites were formed by deformation induced ferrite transformation and dynamic recrystallization of ferrite. Ferrite grains formed by heavy deformation grew very fast during holding time after compression. It was also found that ferrite grains became coarser with decreasing strain rate.

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In the present investigation, the hardness of low carbon microduplex steels consisting of lower bainite and martensite was evaluated as a function of the volume percent of lower bainite. In addition, a series of the microstructural examination was carried out in detail through optical microscopy. SEM and TEM in order to elucidate the effect of the microduplex microstructure of these steels on the hardness. The present investigation was conducted as one of the preliminary studies to find the new steel microstructure for bolt which can exhibit the enhanced strength and delayed fracture resistance simultaneously without major compositional modification. The microstructural characteristics of the present microduplex low carbon steels were manifested by the presence of the needle type lower bainite partitioning the prior austenite grains. Hardness measurement, of these steels showed that, without tempering, there existed an optimum volume fraction of lower bainite showing maximum hardness. The hardness was higher than that of martensite single phase. However, after tempering, a definite hardness peak was not observed but hardness of tempered microduplex steels was somewhat higher than that of tempered martensite single phase up to 70% of lower bainite volume fraction. The analysis of the microstructure and phase transformation characteristics revealed that the strengthening of lower bainite - martensite microduplex structure was attributed to (a) the refinement of martensite lath due to the partition of the prior austenite grains by lower bainite and (b) increase of martensite hardness due to carbon redistribution during austenite - lower bainite transformation.

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Changes in microstructure and toughness at the heat affected zone after welding thermal cycle simulations have been investigated in steels containing titanium oxide and aluminum inclusion particles. The titanium oxide and aluminum inclusion particles acted as preferential nucleation sites for intragranular ferrite plates within prior austenite grains. The HAZ toughness of the steels containing titanium oxide particles was over 100 joule at -20℃ test temperature, while that of the steels containing aluminum inclusions was over 300 joule at the same condition. Consequently, the proper titanium oxide and aluminum inclusion particles accelerated the formation of fine ferrite grains inside prior austenite grains, resulting in a remarkable improvement of heat affected zone toughness.

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