발간논문

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Systematic microstructure control of low carbon steels processed by the continuous annealing could improve formability. In the present study, microstructure evolution including grain size, carbides and MnS(or (Fe, Mn)S) precipitates in hot coiled and annealed low carbon Al killed steel sheets was investigated as the function of Mn content and hot coiling temperature. r￣-value and mechanical properties were measured to check the formability of the steels. Results showed that increased r￣-value revealed for the steel containing low Mn and coiled at high temperature due to the coarsened carbides in hot rolled steel sheets. It was found that the improvement of r￣-value is closely related to the development of high intensity of {222} texture and low intensity of {200} and {110} texture, which were obtained with coarsened carbides during the recrystallization stage. Lowering Mn content might improve the strength of low carbon Al-killed steel as a result of grain refinement by fine Mn-sulfides precipitation. However there exists the lower limit of Mn content of the steels to prevent hot rolled edge cracks and to maintain higher elongation. Optimum Mn content was found about 0.1wt.% for the 0.02wt.%C-0.008wt.%S steels used in this study.

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The grain growth characteristics of dual-phase brass have been investigated using pure Cu, Cu -15%, -30%, -36%, -38%, -40% and -45% Zn brass which are composed of α-, β- and (α+β) dual-phase. The heat treatment has been performed at 600∼800℃ for 1∼100h and the relation between mean grain radius γ￣ and annealing time t can be described as follows : (γ￣)²-(γ￣_0)²=k₂·t (1) In the case of dual-phase brass, the grain growth needs diffusion of alloying elements, because, the chemical composition of α- and β- phase differs each other. The grains of minor phase grow in a mode of Ostwald, ripening while, the grains of major phase are restricted by pinning by the minor phase grains. In the β-rich dual-phase brass, the grain growth is controlled by volume diffusion in the α-phase, and the growth rate is expressed by the equation (2) (γ￣)³-(γ￣_0)³=k₃·t (2) In the α-rich dual-phase brass however, boundary diffusion is predominant over volume diffusion in the β-phase, and the growth rate is expressed as follows : (γ￣)⁴- (γ￣_0)⁴=k₄·t (3)

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The deformation induced martensitic transformation phenomena in 304 stainless steel have been investigated in this study in relation to the inelastic deformation theory. A new kinetics equation for deformation induced martensitic transformation has been formulated based on the inelastic deformation theory as f/f_s=1-exp(-βεⁿ) with the parameter β characterizing the stability of austenite, n denoting a deformation mode parameter, and f_s the saturation value of transformed martensites. The value of n was determined as n = 2.2 for this steel, while the stability parameter β was found to be inversely proportional to the test temperature. Consequently the M_d temperature, above which martensitic transformation can not occur regardless. of the amount of plastic deformation, can be determined indirectly as the temperature with β=0. The true stress-true strain curves at lower temperatures showed low work hardening rate region followed by rapid increase in work hardening rate, while the curves at near room temperature showed a continuously decreasing low work hardening rate. As the test temperature increased, the maximum transformation rate (df/dε) decreased, but the inelastic strain Corresponding to the peak transformation rate increased resulting in a substantial ductility enhancement.

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Effects of alloying elements such as Ca and P on the microstructure of squeeze cast Mg-Al-Zn-Si alloys have been investigated in the present study. Addition of Si in Mg-Al-Zn alloys results in the formation of Mg₂Si in two types of morphologies; polygonal type and chinese script type. With increasing Al content the formation of polygonal type Mg₂Si is pronounced. Addition of Ca results in the homogeneous distribution of fine polygonal Mg₂Si particles. Ca is segregated mainly around the secondary phase such as Mg_(17)Al_(12), P provides the nucleation sites for Mg₂Si, results in the homogeneous distribution of the fine polygonal Mg₂Si particles. Accordingly, Ca and P containing Mg-Al-Zn-Si alloys have the much finer microstructure than those with no Ca and P since the fine polygonal Mg₂Si particles restrict the grain growth of Mg alloys during squeeze casting and subsequent heat treatment.

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The present study was to investigate the effects of alloying elements on the high temperature deformation when Cu, Si, Mn and(or) Cr were added in the base 6063 alloy within the chemical composition range of 6063 alloy. The statistical 2^(4-1) factorial design was adopted to study the high temperature deformation. The compression test was carried out in the temperature range of 350∼500℃ and the peak stress for deformation of various 6063 alloys was obtained. The results represent that Cu increases the peak stress, whereas Cr decreases the peak stress below 450℃ but increases the peak stress above 450℃. Si and Mn decreases the peak stress below 450℃ and increases the peak stress above 450℃.

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In order to investigate the cause of increase in δ-ferrite content from 9.1 to 14.3% with increasing the Cr_(eq)/Ni_(eq) ratio from 1.48 to 1.71 in type 309S stainless steel, the solidification path and transformation of δ-ferrite to austenite were studied by interrupted quenching experiment of unidirectional solidification.δ-ferrite in 309S stainless steel precipitates with primary dendrite irrespective of Cr_(eq)/Ni_(eq) ratio but the formation mechanism of austenite is different depending on Cr_(eq)/Ni_(eq) ratio. Stainless steel at 1.48 of Cr_(eq)/Ni_(eq) starts to form austenite from liquid remained at interdendrites at 1397℃ and completes solidification at 1357℃, but stainless steel at above 1.48 of Cr_(eq)/Ni_(eq) forms austenite by peritectic reaction before solidification. As the Cr_(eq)/Ni_(eq) ratio increased from 1.48 to 1.71, maximum δ-ferrite content formed before austenite formation increased from 54 to 75% and austenitic formation temperature decreased from 1390 to 1383℃.

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Transgranular stress-corrosion cracking (T-SCC) in disordered Cu-25 at% Au single crystals was studied in 0.6 M NaCl and 0.13 M FeCl₃ solutions. This system is especially significant because cleavage-like fracture can occurqr in a normally ductile fcc system where the dominant surface process is de-alloying. De-alloying, i. e., selective dissolution of a less noble component, is believed to play an important role in embrittlement of Cu-25 at% Au during T-SCC. Therefore steady-state polarization, scratching, bending, zero imposed strain-rate tensile tests, and slow strain-rate tensile tests were carried out to establish the relationship between de-alloying and T-SCC. Under slow strain-rate loading, effects of potential and orientation on crack-propagation were also considered. The instantaneous and the average crack velocities were directly obtained by using the one-to-one correlation between current-transients/load-drop events resulting from discontinuous crack growth, and crack-arrest markings appearing on the fracture surface. Two independent methods using current transient or load-drop were used to obtain the values of the instantaneous and the average crack velocities. The result showed that the instantaneous crack velocity is faster than the average crack velocity by approximately three orders of magnitude. In addition, the instantaneous crack velocity is much faster than that expected by the slip dissolution model but much slower than that expected for brittle cleavage.

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The solid particle erosion behavior of Fe-25 at.% Al iron aluminides(Fe₃Al) at room temperature has been investigated. It was found that the erosion rate of the aluminide increases as the impingement angle decreases, which is ductile erosion behavior. SEM observations revealed that lip formation and its subsequent detachment from the surface is the major erosion mechanism of the aluminide. Aluminides with the B2 structure showed higher erosion resistance than those with the DO₃ structure, however, the rate difference was not significant. The different erosion resistances among the aluminides with different phases were explained by the strain-hardening-rate differences.

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To clarify the effects of microstructure on the change of mechanical properties and monotonic tensile fracture mode in the Cl alloy aged at 180℃, the detailed measurement of hardness, tensile strength, elongation and the observation of optical micrography, scanning electron micrography, transmision electron micrography and energy dispersive X-ray analysis has been carried out. In the peak aged, the Cl alloy has much better tensile strength than other alloys, such as 2090, 8090, 2020 and 7075 alloy but has lower elongation than 7075 and 2090 alloys. The fracture surface of Cl alloy show a transgranular dimple fracture mode in the under aged and show a sub-grain boundary intergranular fracture mode caused by the secondry crack along the sub-grain boundary in the peak aged. The Cl alloy, in the fracture surface, has (Al_xCu_yAg_z), (Al_xCu_yFe_z) and (Cu_xAl_yAg_z) type inclusions and U {T₁(Al₂CuLi) or Ω(?)} phase at the grain boundary, which strengthen the matrix.

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Fe-15Cr-5V-X%C(X=0.75, 1.2 and 2.3) alloys have been poured into sand mold, and quenched interruptly to study the solidification process and microstructures varied with carbon content. Solidified structures of Fe-15Cr-5V-X%C alloys were made of α-ferrite +MC+M_7C₃ in 0.75%C, α-ferrite+MC+M_(23)C_6+M_7C₃ in 1.2%C and α+MC+M_7C₃ in 2.3%C alloy. Hardness increased with increasing carbon content and wear resistance was dependent on the existance of eutectic M_7C₃ carbide. Eutectic was the most brittle structure in the solidified structures and primary phase supported the strength of Fe-15Cr-5V-X%C alloys.

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