• Corrosion Science and Technology
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• v.9 no.2
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• pp.74-80
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• 2010

For the purpose of applying a hot-dip Zn-6mass%Al-3mass%Mg alloy coated steel sheet (ZAM) to automotive body materials, a laboratory study of the general properties required for inner and outer panels of automotive bodies was performed. Even with only light coating weight, ZAM showed an excellent corrosion resistance in terms of both cosmetic and perforation corrosion compared to the currently used materials for automotive bodies, GI70 and GA45. In our study, it was confirmed that ZAM exhibits as good as or better properties than GI70 in terms of spot weldability and press formability. Furthermore, since the same corrosion resistance can be achieved with less coating weight by applying ZAM, laser weldability is better than GI and GA.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2001.11a
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• pp.13-13
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• 2001

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2003.10a
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• pp.38-39
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• 2003

• Korean Journal of Metals and Materials
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• v.50 no.6
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• pp.433-438
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• 2012

Strength change in ultra low carbon steel carburized at $880^{\circ}C$ and $930^{\circ}C$ for 10, 30, 60 and 120 minutes was investigated. The results were analyzed by a tensile test, chemical composition analysis, optical microscopy and scanning electron microscopy. Stress in the 0.5% strain specimen in the tensile test increased as the time treated at $880^{\circ}C$ and $930^{\circ}C$ increased, because the carbon diffusion layer and the martensite of the specimen increased with increasing treatment time. Martensite was found in the ferrite region in the specimen treated at $880^{\circ}C$, which is attributed to grain boundary diffusion.

• Korean Journal of Metals and Materials
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• v.50 no.12
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• pp.883-889
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• 2012

Carbon diffusion of ultra low carbon steel treated at $880^{\circ}C$ and $930^{\circ}C$ for 10, 30, 60 and 120 minutes was investigated using optical microscopy, SAM, EPMA, and Micro Vickers. The martensite patterns of the specimens treated at $880^{\circ}C$ and $930^{\circ}C$ were different. Martensite in the ferrite region was found in the specimen treated at $880^{\circ}C$ because of grain boundary diffusion. Such phenomena is explained by a carbon diffusion model.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.218-221
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• 2008

Hot Press Forming (HPF), an advanced sheet forming method in which a high strength part can be produced by forming at high temperature and rapid cooling in dies, is one of the most successful forming process in producing components with complex geometric shape, high strength and a minimum of springback. In order to obtain effectively and accurately numerical finite element simulations of the actual HPF process, the flow stress of a boron steel in the austenitic state at elevated temperatures has been investigated with Gleeble system. To evaluate the formability of the thermo- mechanical material characteristics in the HPF process, the FLDo defined at the lowest point in the forming limit diagrams of a boron steel has been investigated. In addition, the simulation results of thermo-mechanical coupled analysis of an automobile one-piece lower-arm part are compared with the experimental ones to confirm the validity of the proposed simulations.

• Transactions of Materials Processing
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• v.21 no.3
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• pp.186-194
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• 2012

Hot stamping is a forming method that offers various advantages such as superior mechanical properties, good formability, and very small springback. However, relatively large-sized parts, such as front pillars, exhibit poor formability when hot stamped due to the limited material flow and thickness reduction imparted by the process. This reduction in thickness can also lead to cracks. One of the reasons is the relatively high friction between the sheet and the die. In this study, in order to obtain the optimal conditions for hot stamping of front pillars, various process parameters were studied and analyzed using the sheet forming software, J-STAMP. The effects of various parameters such as the die structure, blank shape, blank holding force, punch speed, clearance(upper and lower dies) and distance block were analyzed and compared.

• Corrosion Science and Technology
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• v.11 no.1
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• pp.29-36
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• 2012

The resistance spot welding of high strength steel degrades the weldability because of its high strength with rich chemical composition and coating layer to protect from corrosion. During the each resistance welding process the electrodes tip reacts with coating layer, then subsequently deteriorates and shorten electrode life. In this study, the Al-coated HPF (Hot Press Forming) steels and Zn-coated TRIP steels were used to investigate the electrode life for resistance spot welding. Experimental results show that the reactivity of Al-coating on HPF steels to electrode tip surface behaviors different from the conventional Zn-coated high strength steels. The electrode tip diameter and nugget size in electrode life test of Al-coated HPF steels are observed to be constant with respect to weld numbers. For Al-coated HPF steels, the hard aluminum oxide layer being formed during high temperature heat treatment process reduces reactivity with copper electrode during the resistance welding process. Eventually, the electrode life in resistance spot welding of Al-coated HPF steels has the advantage over the galvanized steel sheets.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.10
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• pp.1221-1226
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• 2012

The hot-stamping technique is a forming method used for manufacturing high-strength parts, in which a part is cooled rapidly after press forming above the austenite transformation temperature. Boron steel, which contains a very small amount of boron, is one of the materials used for hot stamping. The purpose of this study is to investigate the mechanical properties of boron steel according to the heat-treatment conditions and the formability by using an Erichsen cupping test. Die quenching from various temperatures was conducted for different elapsed heat-treatment times. Laser-welded boron steel after quenching at 1173 K-0 s has a tensile strength of 1203 MPa. This is 79% of the tensile strength of the base metal (1522 MPa). The formability of boron steel was not significantly different from that at the mold temperature. However, it decreased with increasing forming speed. These properties provide practical information for the use of boron steels for hot stamping.

• Transactions of Materials Processing
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• v.33 no.3
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• pp.161-168
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• 2024

Aluminum alloy sheets, compared to conventional steel sheets, face challenges in press forming due to their lower elongation. To enhance their formability, extensive research has focused on forming technologies at elevated temperatures, specifically warm forming at around 300℃ and hot forming at approximately 500℃. This study proposes that the formability of aluminum alloy sheets can be significantly enhanced using a multi-stage hot forming technique. The research also investigates whether the strength of the A6061 aluminum alloy, known for its precipitation hardening, can be maintained when formed below the precipitate solid solution temperature. In the experiments, the A6061-T6 sheet underwent heating and rapid cooling between 250 and 500℃. The mechanical properties were evaluated at each stage of the process. The findings revealed that when the initial heat treatment was below 350℃, the strength of the material remained unchanged. However, at temperatures above 400℃, there was a noticeable decrease in strength coupled with an increase in elongation. Conversely, when the secondary heat treatment was conducted at temperatures of 350℃ or lower, the strength remained comparable to that of the initial heat treated material. However, at higher temperatures, a reduction in strength and an increase in elongation were observed.