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Temperature and phase transformation behaviour during cooling process in hot strip rolling is very important, because this determines the mechanical properties of hot-rolled steel products. This paper describes a development of a temperature calculation model based on the finite element method for a cooling process in hot strip rolling. The model was coupled with a thermodynamic analysis of Fe-C-Mn-Si system using a two-sublattice model and a phase transformation kinetics model using JMA equation and the additivity rule. Using the developed model, temperature and phase histories of hot-rolled steels on run-out table(ROT) and during air-cooling after coiling were calculated. From these analyses, suitable cooling pattern on ROT to produce quality hot coils such as TRIP and high carbon steels could be designed.

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This paper describes the bending fatigue properties of the high-carbon steel wire containing around 0.6 wt.% C, which is used as a stranded wire for ACSR(Aluminum Stranded Conductors Steel Reinforced) in transmission power lines. ACSR which consists of galvanized steel strand and aluminum one always experiences serious vibration by wind. Therefore, it is very important to clarify the fatigue property of core wire for ACSR to determine its lifetime and stability of power delivery. Since steel wires are produced by wire drawing process, they possess a very long fiber lamellar microstructure which has a very short inter-layer distance between ferrite and cementite layer. The long fiber lamellar microstructure is expected to have a different fatigue behavior compared to conventional plate and bulk-type materials. In order to characterize the fatigue property of cold-drawn high-carbon steel wires, bending fatigue tests were carried out on specimens drawn to various strains. The fatigue strength was about 38 kgf/㎟ at ε= 1.19, which was higher than the required strength of 32 kgf/㎟. This is probably due to the very fine elongated pearlitic microstructure formed by the drawing process, which suppresses formation and growth of microcracks. The several unique characteristics of drawn wires are also discussed.

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Effect of oxygen and iron on the high temperature deformation behaviors of CP-Ti was studied at temperatures from 750 ℃ to 950 ℃ using compression test at various strain rates from 0.001s^(-1) to 10s^(-1). High temperature deformation efficiencies and flow instability conditions were, in particular, evaluated from their high temperature flow curves. The work hardening rates were always higher in the grade 1 than in the grade 4 CP-Ti. This was mainly attributed to the difference in the range of α+β two-phase field arising from the different Fe content. The optimum working condition was estimated to be the strain rate of 10s^(-1) at 850℃ and to be the strain rate of 10s^(-1) at 900℃ for grade 1 and grade 4 CP-Ti, respectively. The dynamic recrystallization mechanism was believed to be mainly responsible for the high deformation efficiency for both grades. The increase of oxygen content shifted the condition for the high deformation efficiency toward a higher temperature or lower strain rate condition, in a good agreement with the theoretical expectation for the shift direction of the condition for dynamic recrystallization. At the intermediate strain rates and at about 900℃, power dissipation efficiencies were very low in both specimens. This was believed to be closely related to the occurrence of α→β transformation during deformation.

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The present study investigated the effect of N addition on the strength characteristics of 0.1%C-1%Mn-0.5%Mo-0.02%Ti fire-resistant steel for construction with low yield ratio. 0.1%C-1%Mn-0.5%Mo-0.02%Ti(A) and 0.1%C-1%Mn-0.5%Mo-0.02%Ti-0.015%N(B) specimens showed bainite phase in matrix of ferrite and pearlite phase, the addition of N suppressed the transformation of pearlite and accelerated transformation to bainite. Yield strength(Y.S) ratio, yield and tensile strength of B specimen was higher value than those of A specimen, but yield ratio(Y.R) value was lower. In the two specimens treated by air cooling or quenching and tempering after rolling, yield strength and Y.S ratio decreased slowly with increasing test temperature, and rapidly decreased at over 600℃. But Y.S ratio at 600℃ was higher than 70%. In case of specimen tempered after quenching, yield and tensile strength were higher than that of air cooling specimen but Y.S ratio was lower. With increasing finish rolling temperature, volume fraction of ferrite and pearlite phase was decreased, and volume fraction of bainite and Y.S ratio was rapidly increased, while Y.R value was unchanged. In two specimens, Y.S ratio was strongly controlled by volume fraction of second phase, but Y.R value and yield strength were affected by other factors than the volume fraction of second phase.

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The effect of Ni-content(8.3∼12 wt%) on the tensile properties and work hardening behavior between 20℃ and -196℃, formability and weldability was studied on Type 304 stainless steel used for the membrane of LNG storage tank. At room temperature, the hardening and ductility indices (such as tensile strength, strain hardening exponent and elongation) were increased and formability was improved by decreasing the Ni-content. When the Ni-content is less than 10 wt.%, lower yield point is observed at temperatures lower than -60℃. It is due to the dynamic strain softening or transformation plasticity, which accompanies the rapid increase in the amount of strain induced martensite(α`) at low strains. Dynamic strain softening or transformation plasticity is not observed when only E-martensitic transformation occurs at low strains. Regardless of the Ni-content in the Type 304 stainless steel, no cracks were observed during the fabrication of the LNG storage tank membrane since the stain level of the maximum deformed area was the half of forming limit curve. During the gas-tungsten-arc welding, the variation of Ni-content did not affect the penetration characteristics of the Type 304 stainless steel.

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The effects of nitrogen on the creep properties of type 316L(N) stainless steels with three different nitrogen contents from 0.04% to 0.15% were investigated. Creep tests were carried out using constant-load single lever machines at 550∼650℃ in the air. The time to rupture increased and the minimum creep rate decreased with the addition of nitrogen. At constant stress, the rupture elongation decreased with the addition of nitrogen. Intergranular and transgranular fracture mode were mixed in all specimens. Cavity and carbides were nucleated at grain boundary and the number of cavity and carbide at constant stress was increased with the addition of nitrogen because of the increase in the time to rupture and carbide precipitation due to the addition of nitrogen. The increase of rupture time with the addition of nitrogen for type 316L(N) stainless steel was attributed to the combined effect of the decrease of minimum creep rate due to the increase of tensile strength and the rupture elongation due to the precipitation at grain boundaries.

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Microstructure and creep-rupture property of Alloy 713LC were investigated to optimize the utility of Alloy 713LC. The applied heat treatment condition was 1170℃/2hours/air cooling followed by 650℃/16hours/air cooling. During the heat treatment at 1170℃, MC carbide on grain boundary of as-cast Alloy 713LC was partially dissolved and coarsening of γ` particles was contemporaneously taken place so that inhomogeneous distribution in size of γ` particles was observed in heat-treated Alloy 713LC. It was concluded from the high temperature tensile test that the equi-cohesive temperature(ECT) of Alloy 713LC is around 750℃. At higher temperature than ECT, stress-rupture time was not changed after heat treatment. On the other hand, at the temperature around ECT, stress-rupture time decreased by the factor of 10 after heat treatment. This was explained by relating it to the microstructural change of grain boundary and grain interior caused by heat treatment.

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In this study, microstructural investigation was conducted on the microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the microcracked regions showed that most of the microcracks were formed mainly along tortoiseshell-shaped depressions and that the number and size of microcracks were considerably reduced when strip casting was done right after shot-blasting or pickling treatment of the cast roll surface. This microcracking phenomenon was closely related with the formation of a black oxide layer, which was mainly composed of manganese-rich oxide, on the roll surface. The black oxide layer acted as a barrier of thermal transfer between a roll and melt, led to an increased gas gap and inhomogeneous solidification of cast strips, and thus played a role in forming both tortoiseshell-shaped depressions and microcracks. The installation of brush rolls behind cast rolls was suggested as a method to prevent microcracks, since the brush rolls could continuously scrape the black oxide layer from the roll surface.

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Effects of both Al concentration in molten Zn and galvannealing temperature on the microstructure and defoliation properties of the galvannealed coating in continuous galvanizing line were investigated by scanning electron microscopy, X-ray diffraction and deep drawing test. The amount of powdering, a measure of defoliation, increased with increasing the Al concentration and was associated with galvannealing temperature. Below 0.135 wt% Al, coating surface consists of the mixture of δ_(1p) and ζ phases. The thickness of Γ phase was measured to be lower than 1 ㎛. It is considered that the formation of relatively ductile ζ phase and thin Γ phase contributes to improve the press formability, i.e. the decrease of the amount of powdering. Above 0.155 wt% Al, coating surface shows the mixture of δ_(1p) and δ_(1k) phases. The appearance of brittle δ_(1k) phase approximately 10 ㎛ in size can account for the high amount of defoliation during deep drawing. The optimum Al concentration in Zn bath of continuous galvanizing line was found to be 0.135 wt% with galvannealing temperature below 520℃.

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Deformation history of the materials during equal channel angular pressing(ECAP) is calculated by FEM. The deformation behavior during ECAP is presented in terms of stretch ratio and shear angle with respect to those of simple shear deformation. Although many researchers have presented that the uniform and simple shear deformation during ECAP can be achieved, the material subjected through the ECAP die is shown to experience not a simple shear deformation but a complex deformation in this FEM analysis.

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