• Corrosion Science and Technology
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• 제23권3호
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• pp.221-227
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• 2024

During hot stamping of hot-dip zinc-coated steel sheets such as hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets, an oxide mainly composed of ZnO is formed on the sheet surface. However, excessive formation of ZnO can lead to a decrease in the amount of metal Zn in the coating layer, decreasing the corrosion resistance of hot-stamped members. Therefore, it is important to suppress excessive formation of ZnO. While the formation of Al oxides and Mn oxides along with ZnO layer during the hot stamping of hot-dip zinc-coated steel sheets can affect ZnO formation, crystal structures of such oxides have not been elucidated clearly. Thus, this study aimed to analyze structures of oxides formed during hot stamping of hot-dip galvannealed steel sheets using transmission electron microscopy. Results indicated the formation of an oxide layer comprising ZnAl₂O₄ at the interface between ZnO and the coating layer with Mn₃O₄ at the outermost of an oxide layer.

• Corrosion Science and Technology
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• 제9권3호
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• pp.109-115
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• 2010

Interstitial-free (IF) steels are widely used for car body material. However, a few types of streaky mark defect are commonly found on hot dip galvannealed (GA) IF steel sheets. In the present study, both the phase structure of a streaky mark defect and the microstructure of the substrate just below it were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that the bright streaky mark area was composed of ${\delta}$ phase while the dark normal area was full of craters. More than half of the grains at the uppermost surface of the substrate just below the streaky mark defect are unrecrystallized grains which could result from lower finish rolling temperature during hot rolling and be kept stable during the annealing process, while almost all the grains in the normal area are equiaxed grains. In order to confirm the effect of the unrecrystallized grains on the coating morphology, hot dip galvannealing simulation experiments were carried out in IWATANI HDPS. It is proved that the unrecrystallized grains accelerate the Fe-Zn reaction rate during galvannealing and result in a flatter coating surface and an even coating thickness. Finally, a formation mechanism of the streaky mark defect on the hot dip galvannealed IF steel sheet was discussed.

• 한국표면공학회지
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• 제32권2호
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• pp.134-143
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• 1999

The effects of silicon on galvannealing behavior of interstitial-free (IF) steels were studied. The growth rate of the Fe-Zn alloy layer was retarded as silicon in the steel added. Titanium in steel strongly favors Fe-Zn reaction, in particular outburst structures, whereas silicon inhibit them. Cross-sectional and planar views of galvannealed coatings were investigated to characterize alloy phase development. A possible mechanism to explain the retardation effect of silicon is discussed in terms of concentration on surface and inhibition layer.

• 한국자동차공학회논문집
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• 제16권6호
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• pp.141-147
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• 2008

Hot-dip galvannealed sheet (GA) with high strength of 590MPa grade in tensile strength, has developed for automotive applications. However, for a successful application, the microstructure and galvannealing behavior of galvannealed TRIP steel sheets must be strictly controlled. High silicon contents steel has problems with weld-ability, zinc coating and reduction of retained austenite volume fraction after galvannealing process. The main purpose of this study is to solve the problem as indicated above.

• 대한금속재료학회지
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• 제46권2호
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• pp.58-64
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• 2008

Surface-void defects observed on the galvannealed(GA) steel sheets in Interstitial-free high-strengthened steels containing Si and Mn have been investigated using the combination of the FIB(Focused Ion Beam) and FE-TEM(Field Emission-Transmission Electron Microscope) techniques. The scanning ion micrographs of cross-section microstructure of defects showed that these defects were identified as craters which were formed on the projecting part of the substrate surface. Also, those craters were formed on the Si or Mn-Si oxides film through the whole interface between galvannealed coating and steel substrate. Interface enrichments and oxidations of the active alloying elements such as Si and Mn during reduction annealing process for galvanizing were found to interrupt Zn and Fe interdiffusion during galvannealing process. During galvannealing, Zn and Fe interdiffusion is preferentially started on the clean substrate surface which have no oxide layer on. And then, during galvannealing, crater is developed with consumption of molten zinc on the oxide layer.

• 한국재료학회:학술대회논문집
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• 한국재료학회 2010년도 춘계학술발표대회
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• pp.40.1-40.1
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• 2010

Recently high strength steel sheets with high formability for automotive parts have been being developed to meet the demands for passenger safety and weight reduction of car body. Among these high strength steels, dual-phase steels are regarded as one of the attractive steels due to their excellent mechanical properties including high strength and ductility. However, to be successfully applied to automotive parts they should be corrosion resistant enough to satisfy the required quality of car maker. This also requires their feasibility for galvannealed production including hot dip galvanizability. In this study has been placed on understanding the effects of heat-treatment(austenizing and isothermal treatment) on the microstructures and mechanical properties of a 0.06C-0.03Si-2.0Mn high strength steel for cold forming. The microstructure and phase distribution were examined with eth aids of SEM, EBSD, TEM etc.. Through the study the production of 590MPa grade DP GA steels with good formability and galvaniability were shown to be possible.

• 한국표면공학회지
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• 제37권5호
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• pp.263-272
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• 2004

High strength steels such as transformation induced plastic steel, dual phase and solid solution Hardening have been developed and continuously improved due to the intensified needs in the automotive industry. But silicon and manganese in transformation induced plastic steels were known to exhibit harmful effects on galvannealing reaction by oxide film formed during heat treatment. Therefore, in this work, the applicability of Zn-Ni electrodeposition instead of hot dip galvannealed coating to transformation induced plastic steels was evaluated and optimum electroplating condition was investigated. Based on these investigations optimized electroplating conditions were proposed and Zn-Ni electrogalvanized steel sheet was produced by EGL (electrogalvanized line). Its perfomance properties for automotive steel was evaluated.

• 대한기계학회논문집A
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• 제32권12호
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• pp.1107-1114
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• 2008

This paper is aimed to understand the effect of different galvannealing temperatures on the frictional properties and Fe-Zn intermetallic phases of the galvannealed (GA) coatings on steel sheets. Their galvannealing treatments were conducted at 465, 505, 515 and $540^{\circ}C$ for about 10s in the additional heating furnace of an industrial continuous hot-dip galvanizing line. The mechanical and the frictional properties of the coatings were estimated using nanoindentation, nanoscratch, micro vickers hardness tests and flat friction tests, which were performed at contact pressures of 4, 20 and 80MPa. Also, the correlation between the microstructure and the frictional properties of the GA coatings were investigated by SEM observation for the cross-section of the GA coating after and before flat friction tests. The results showed that the mechanical and the frictional properties of the coatings are strongly dependent on their phase distributions and microstructure. Especially, in low contact pressure of 4MPa the frictional properties of the coatings were dependent on the surface phases and morphology, while in high contact pressure of 80MPa it was influenced by their mechanical properties based on the dominant phase distributions.

• 한국표면공학회지
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• 제34권4호
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• pp.297-302
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• 2001

As GA (Hot dip galvannealed steel sheet) has good corrosion resistance, weldability and paintability as well as excellent formability, it's demand is rapidly increasing for automotive panels. The GA coated layers are composed of several kinds of brittle Fe-Zn Metallic compounds which are susceptible to powdering during the press forming, however, very careful controls of manufacturing conditions such as galvannealing heat-treatment or bath composition are essential to meet with the required quality of automotive use. In this study the required characteristics of automotive panel are practically surveyed in detail and the appropriate manufacturing conditions of galvannealing or bath composition have experimentally investigated by using the various analyzing and simulating equipments. The result in this study indicated that the corrosion resistance at the cut edges was improved by increasing of coating weight and decreasing of thickness of sheet steels.

• Corrosion Science and Technology
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• 제2권5호
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• pp.247-251
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• 2003

The corrosion forms of automotive body panels are various. One of the representations is a corrosion pitting and its propagation on the lapped portion by galvanic corrosion. But it has been difficult in correlation analysis about the corrosion propagation rate and mechanism of pitting and the actual automotive body in field. This present study interprets experimentally the rust pitting occurrence mechanism on the lapped panels through experimental methods. And field car investigation was executed for correlation analysis with experimental results. This paper compares corrosion propagation rate by pitting on hot-dip galvannealed steel sheets with corrosion forms in the automotive field condition. The research fundamentals which make it possible to predict the pitting occurrence and propagation on the lapped panels in the actual vehicles are given.