• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.1
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• pp.171-176
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• 2007

An important challenging issue in the automotive industry is the light-weight, safe design and enhancement of crash response of an auto-body structures. These objectives lead to increasing adoption of high strength steel sheet for inner and outer auto-body members. This paper evaluates the dynamic tensile characteristics of high strength steel sheets, HS45R, TRIP60, DP60 and DP100, along the rolling direction and transverse direction. Static tensile tests were carried out at the strain rate of 0.003/sec using the static tensile machine (Instron 5583). Dynamic tensile tests were carried out at the range of strain rate from 0.1/sec to 200/sec using a high speed material testing machine developed. The tensile tests acquire stress-strain relation and strain rate sensitivity of each material. The experimental results show two important aspects for high strength steels: the flow stress increases as strain rate increases; the strain hardening decreases as the tensile stress increases. The experiments also produce interesting results that the elongation does not decrease even when the strain rate increases.

• Corrosion Science and Technology
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• v.23 no.2
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• pp.83-92
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• 2024

A hot-dip simulator was utilized to replicate abnormal coating buildup observed during line stops at galvanize lines, assessing the influence of processing conditions on buildup (up to 14 mm/side). Steel samples from 19 coils (comprising IF, BH, LCAK, HSLA, DP600-DP1180, Si: 0.006 - 0.8 wt%, P: 0.009 - 0.045 wt%) were examined to explore the phenomenon of heavy coating growth. It was discovered that heavy coating buildup (~3 mm/h) and rapid strip dissolution (~0.17 mm/h) in a GA or GI pot can manifest with specific combinations of steel chemistry and processing conditions. The results reveal the formation of a unique coating microstructure, characterized by a blend of bulky Zeta crystals and free Zn pockets/networks due to the "Sandlin" growth mechanism. Key variables contributing to abnormal coating growth include steel Si content, anneal temperature, dew point in heating and soaking furnaces, Zn pot temperature, Zn bath Al%, and cold-rolling reduction%. At ArcelorMittal Dofasco galvanize lines, an automatic online warning system for operators and special scheduling for incoming Si-bearing steels have been implemented, effectively preventing further heavy buildup occurrences.

• Transactions of Materials Processing
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• v.28 no.6
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• pp.368-374
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• 2019

The past recent years have seen an increasing use of high-strength steel sheets in the automotive industry. However, the formability and damage prediction of these materials requires accurate acquisition of necking and fracture strains. Digital image correlation (DIC) is used to accurately capture the necking and fracture strains during testing. The fact that single time points of capturing vary with frame rate makes the need for an investigation necessary. For the high-strength steel DP980, the frame-rate dependences of the final necking and fracture strains values are analyzed here. To eliminate the influence of gauge length, the strains were measured locally by DIC. Results for three specimen shapes obtained with frame rates of 1 and 900 fps (frames per second) were considered and based on them, triaxiality failure diagrams (TFD) are established. It was observed that after diffuse necking, the deformation path departed from the initially linear one, and the stress triaxiality grew with ongoing deformation. It was further revealed that the frame rate-dependence of the necking strain was rather low (< 2%), whereas the fracture strain could be underestimated by up to 8% when the lower frame rate of 1 fps was used (compared with 900 fps). In this study, this issue is investigated while taking into consideration the three different triaxialities. These results demonstrate the importance of choosing an appropriate frame rate for the determination of necking and fracture strains in particular.

• Journal of the Korean Institute of Gas
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• v.18 no.1
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• pp.61-67
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• 2014

Small punch(SP) tests were performed on high strength Dual Phase(DP) steels in order to evaluate the behavior of hydrogen embrittlement. For this purpose, three different kinds of DP steel specimens were charged with hydrogen by electochemical hydrogen charging experiment. After charging with hydrogen, the amount of charged hydrogen was measured. The measurement results showed that amounts of charged hydrogen were largely dependent on the martensite volume fraction of DP steel. The hydrogen charging time of 25 hrs with current densities of 150 and $200mA/cm^2$ was investigated as saturation condition with hydrogen. The analysis results on the SP energy and height of SP bulbs after SP tests showed that those were decreased as the amount of charged hydrogen increased. Fractographs of SP bulbs were observed a brittle fracture mixed with quasi-cleavage fractures, layered structures and clear facets.

• Journal of Welding and Joining
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• v.27 no.2
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• pp.69-75
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• 2009

The effects of welding speed were investigated on penetration characteristics, defects and mechanical properties including formability test in Nd:YAG laser welded 1000MPa grade DP steels. A shielding gas was not used and bead-on-plate welding was performed with various welding speeds at 3.5kW laser power. Defects of surface and inner beads were not observed in all welding speeds. As the welding speed increased, the weld cross-section varied from the trapezoid having wider bottom bead, through X type, finally to V type in partial penetration range of welding speeds. The characteristic of hardness distribution was also investigated. The center of HAZ had maximum hardness, followed by a slight decrease of hardness as approaching to FZ. Significant softening occurred at the HAZ near BM. Regardless of the welding speed, the weld showed approximately the same hardness distribution. In the perpendicular tensile test with respect to the weld direction, all specimens were fractured at the softening zone. In the parallel tensile test to the weld direction, the first crack occurred at weld center and then propagated into the weld. Good formability over 80% was taken for all welding conditions.

• Transactions of Materials Processing
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• v.17 no.3
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• pp.197-202
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• 2008

The control of springback is very important in sheet metal forming since springback affects the dimensional inaccuracy of product. The object of this study is to design the manufacturing process for the improvement of the performance of seat rail by DP780. The influence of process variables such as bend angle and pad force on the springback has been firstly investigated through the comparison between the results of FE-analysis and trial out for initial design based on designer's experience. The process variables of the initial design have been modified in order to improve the dimensional accuracy of seat rail from the prediction of springback by FE-analysis. It was shown from experiment that the improved design satisfied the required specifications such as the dimensional accuracy and the strength of seat rail.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.43-45
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• 2005

For application of advanced high strength hot-rolled steels (i.e. DP590, DP780) to automotive lightweight chassis parts, various technologies from design to forming test, optimization of welding condition and investigation of coating properties were tried. The target part of this study was automotive rear sub frame and we could make $16.8\%$ weight reduction by reducing the material thickness and optimizing the design. In addition, the formability and weldability of the newly developed AHSS, DP780, were evaluated.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2011.05a
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• pp.193-195
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• 2011

강판재 사용에 있어 수소취성현상은 강판재가 고강도화됨에 따라 그 감수성이 커지는 것으로 알려져 있다. 특히 노출강판재의 경우, 표면으로부터 침투해 들어가는 수소에 의한 수소취화 거동에 대한 연구가 필요하다. 본 연구에서는 고강도 DP강을 시험편으로 하여, 음극전기분해법에 의해 수소를 강제주입시킨 후, DP강 표면하 subsurface zone의 수소취화 거동을 평가하고자 하였다. 미소경도분포 측정결과, 수소주입시간이 증가함에 따라 표면하 subsurface zone의 경도 또한 증가하는 것으로 조사되었다. 이는 수소에 의해 표면하 subsurface zone의 경화가 일어나 DP강의 기계적 특성에 영향을 미친것으로 판단된다. 미소경도분포의 측정결과와 미세조직 관찰결과로부터 수소임계침투깊이를 평가하였고, 이를 통해 DP강내 수소확산에 의한 수소임계침투깊이가 존재함을 확인할 수 있었다.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.252-253
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• 2006

The fabrication of complex-shaped parts out of Co-Cr-Mo alloy and 316L stainless steel by three-dimensional printing (3DP) was studied using two grades of each alloy with average particle size of 20 and $75\;{\mu}m$, respectively. To produce sound specimens, the proper 3DP processing parameters were determined. The sintering behavior of the powders was characterized by dilatometric analysis and by batch sintering in argon atmosphere at $1280^{\circ}$ for 2h. The 3DP process has successfully produced complex-shaped biomedical parts with total porosity of 12-25% and homogenous pore structure, which could be suitable for tissue growth into the pores.

• Korean Journal of Materials Research
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• v.21 no.1
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• pp.50-55
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• 2011

Energy resistance welding (ERW) is a pipe-producing process that has high productivity and low manufacturing cost. However, the high heat input of ERW degrades the mechanical property of the pipe. This study investigates the effect of heat input and alloying elements on microstructure and mechanical properties of ERW pipes. As the heat input increased, the ferrite amount increased. The ferrite amount in the weld centerline was larger than t at in the weld boundary. Medium carbon steels (S45C and K55) having 0.3~0.4wt.% carbon yielded a significant difference of ferrite amount in the weld centerline and weld boundary. High alloyed steels (DP780 and K55) having 1.5~1.6wt.% Mn showed a ferrite rich zone in the weld centerline. These phenomena are probably due to decarburization and demanganisation in the weld centerline. As the ferrite fraction increased, the hardness decreased a little for the S45C steels. In addition, DP780 steels and K55 steels showed that the hardness drops when those steels have a ferrite rich zone. But we demonstrated the good tensile property of the DP780 steels and K55 steels in which Mn is included.