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- 저자 : Bongyoon Kim (검색결과 7 건)
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Bongyoon Kim,Seonghoon Jeong,Seong‑Jun Park,Joonoh Moon,Changhee Lee 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.4
In the automotive industry, lightweight steel has received much attention because steel comprises a signifi cant portion of avehicle’s total weight. Fe–Mn–Al–C steel is a representative lightweight steel due to its high performance and low density. However, there is insuffi cient research into the welding characteristics of Fe–Mn–Al–C lightweight steels. In this study, hotductility tests were conducted on austenitic Fe–30Mn–9Al–0.9C steel in order to understand the welding characteristics(cracking resistance) of the heat aff ected zone. During the on-heating thermal cycle, ductility was altered by a decrease inmicroband induced plasticity (MBIP) (softening) and an increase in dynamic recrystallization (DRX) (softening) as the temperatureincreased. Specifi cally, in the range of 773–1073 K, ductility was fairly degraded because neither MBIP nor DRXtook place. During the on-cooling thermal cycle, ductility behavior was changed by both softening and hardening factors,including formation of brittle (Fe, Mn) 3 Al intermetallic compounds with grain growth and re-solidifi ed grain boundaries. However, the hardening eff ect of precipitated κ-carbide was insignifi cant and might not play a signifi cant role in the hotductility behavior of the lightweight alloy used in this study.
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Characterization of bond line discontinuities in a high-Mn TWIP steel pipe welded by HF-ERW
Park, Gitae,Kim, Bongyoon,Kang, Yongjoon,Kang, Heewoong,Lee, Changhee Elsevier 2016 Materials characterization Vol.118 No.-
<P><B>Abstract</B></P> <P>In this work, the microstructure and defects in a high-frequency electrical resistance welded (HF-ERW) pipe of high-Mn twinning-induced plasticity (TWIP) steel were characterized. The microstructure of the base metal and the bond line were examined using both optical microscopy and scanning electron microscopy. The features of the bond line were similar to those of conventional steel. Simultaneously, the circumferential ductility was evaluated via a flaring test. It was concluded that the deterioration of the circumferential ductility in a high-Mn TWIP steel pipe was caused by irregular shaped oxide defects and a penetrator that had been formed during welding. Specifically, the penetrator, which is composed of MnO and Mn<SUB>2</SUB>SiO<SUB>4</SUB>, was found to be the most influential on the circumferential ductility of the welded pipe. The penetrator was analyzed using both an electron probe micro analyzer and transmission electron microscopy, and the formation sequence of the penetrator was evaluated.</P> <P><B>Highlights</B></P> <P> <UL> <LI> This study focused on applying the HF-ERW process to the seam welding of expandable pipe using TWIP steels. </LI> <LI> For improvement of the circumferential ductility, deterioration factors were characterized. </LI> <LI> Penetrator which would mainly deteriorate the circumferential ductility consisted of round MnO and Mn<SUB>2</SUB>SiO<SUB>4</SUB>. </LI> <LI> Metallurgical evidence of existing theory regarding the mechanism of defect formation during the HF-ERW was characterized. </LI> </UL> </P>
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Jeong, Seonghoon,Kim, Bongyoon,Moon, Joonoh,Park, Seong-Jun,Lee, Changhee Elsevier Sequoia 2018 Materials science & engineering Structural materia Vol.726 No.-
<P><B>Abstract</B></P> <P>This study aims to investigate the effects of κ-carbide precipitation behavior in the heat affected zone (HAZ) in FeMnAlC lightweight steels. Three alloys with different amounts of Al were prepared by vacuum induction melting and hot rolling. After solution treatment, the HAZ samples were simulated by a Gleeble simulator with two heat inputs of 10 and 30 kJ/cm. Microstructural analysis with XRD and TEM were carried out while sub-sized tensile test, hardness test, and V-notched Charpy impact test were performed for investigating the mechanical properties of the base steels and HAZ. The results showed that the mechanical properties and precipitation of κ-carbide within the HAZ were strongly related to the Al content and heat input; the tensile strength and hardness of the HAZ increased as the Al content and heat input increased while elongation decreased. On the other hand, in the Charpy impact test, fracture mode transitions in the HAZ (ranging from ductile fracture to brittle inter-granular fracture) occurred in accordance with the Al content and heat input. The different fracture behavior was explained by TEM results, which showed precipitation behavior of κ-carbide in HAZ. Coherent intra-granular κ-carbide was found to cause a transition from ductile fracture to trans-granular cleavage, and we observed that a severe drop of the impact toughness occurred when partially coherent inter-granular κ-carbide appeared. Therefore, our results illustrate that the HAZ of lightweight steels with the proper Al content can be strengthened with minimal loss of impact toughness due to κ-carbide precipitation during the welding process.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The effect of κ-carbide in the HAZ for FeMnAlC alloys was characterized. </LI> <LI> Relationship between mechanical properties and κ-carbide in HAZ was confirmed. </LI> <LI> Formation of intra- and inter-granular κ-carbide affected impact toughness. </LI> </UL> </P>
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Seonghoon Jeong,Gitae Park,Bongyoon Kim,Joonoh Moon,Seong‑Jun Park,Changhee Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.10
This study investigates the relationship between the mechanical characteristics and microstructural transitions in heat-affectedzone (HAZ) of various austenitic Fe–Mn–Al–C lightweight steels. The analysis was conducted on the simulated HAZ sampleswith various post-weld heat treatment (PWHT) conditions. The base steel was prepared via vacuum induction melting, hotrolling, and solution treatment prior to water quenching. Gleeble simulator was used to manufacture the simulated HAZs, andthe samples were subjected to PWHT with various durations by electric furnace. To understand the effects of thermal historyon the mechanical characteristics, ultimate tensile tests, micro-hardness tests, and cryogenic Charpy impact toughness testswere conducted, and microstructural analyses were conducted by optical microscopy, scanning electron microscopy, X-raydiffraction analysis, and transmission electron microscopy. The experimental results confirmed the influence of Al contenton the austenitic Fe–Mn–Al–C lightweight steels, which promotes κ-carbide precipitation, both in the austenite matrix andgrain boundaries, thereby increasing the strength and hardness while decreasing the ductility and toughness. Each mechanicalproperty showed a linear relationship with the growth kinetics of the κ-carbide particles. However, abnormal coarsening ofthe inter-granular κ-carbide during welding and PWHT caused severe embrittlement, regardless of thermal history, by actingas inter-granular crack propagation channels. The experimental results suggest that Fe–Mn–Al–C lightweight steels shouldbe developed with controlling the alloying elements to overcome HAZ discontinuity and mechanical property degradationoccurring by intra- and inter-granular κ-carbide precipitation.
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Jeong, Seonghoon,Park, Gitae,Kim, Bongyoon,Moon, Joonoh,Park, Seong-Jun,Lee, Changhee Elsevier 2019 Materials science & engineering. properties, micro Vol.742 No.-
<P><B>Abstract</B></P> <P>In this study, we investigated the effect of precipitation during the thermal cycles of age hardening and the welding process in hot rolled austenitic FeMnAlC lightweight steel. After solution treatment, samples were prepared under different age hardening conditions, which was conducted at a temperature of 550 °C in a furnace. Samples representing heat affected zones (HAZs) with three different peak temperature conditions were prepared via a Gleeble simulator. For microstructural analyses, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy (TEM) were carried out. The mechanical characteristics were investigated by tensile, hardness, and cryogenic Charpy impact tests. The results showed that precipitation behavior in the lightweight steels had specific influence in mechanical properties in age hardening and welding process. The age hardening effect was shown with increased tensile strength with loss of ductility. In the solution treated specimen, increased hardness appeared in HAZ compared to the base steel, while HAZ softening occurred in every age-hardened specimen. In the cryogenic Charpy impact test, all of the samples except the solution-treated base steel showed negligible energy absorption with inter-granular fracturing. The trends of the experimented HAZ mechanical properties revealed that the precipitation behavior of κ-carbide was strongly related to the overall mechanical properties. Moreover, we found that the welding process removed the hardening effect of aging as a result of drastic κ-carbide dissolution in the high-temperature HAZ regardless of age hardening conditions. The TEM analysis confirmed that all HAZ simulated specimens showed similar precipitation conditions for all cases. Based on our analysis, we confirmed that the precipitation behavior of κ-carbide was the main factor of different HAZ mechanical properties. We also suggested that discontinuous HAZ mechanical properties should be considered during the welding process because of κ-carbide dissolution in FeMnAlC lightweight steels.</P> <P><B>Highlights</B></P> <P> <UL> <LI> FeMnAlC alloy was analyzed for various age hardening and HAZ conditions. </LI> <LI> Mechanical properties had specific relationship with κ-carbide in all cases. </LI> <LI> Inter-granular κ-carbide rapidly formed in all heat treatment conditions. </LI> </UL> </P>
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