The effect of minor alloying elements(Si/Mn) ratio in interstitial free-high strength steel(IF-HSS) and dual phase-high strength steel(DP-HSS) containing Si, Mn etc., and dew points in the annealing atmosphere on the surface properties of the galvanized high strength steels for automobile have been ...
The effect of minor alloying elements(Si/Mn) ratio in interstitial free-high strength steel(IF-HSS) and dual phase-high strength steel(DP-HSS) containing Si, Mn etc., and dew points in the annealing atmosphere on the surface properties of the galvanized high strength steels for automobile have been studied by galvanizing simulation, with particular attention towards alloying element enrichment, the occurrence of selective oxidation of the surfaces immediately prior to hot dip galvanizing. Annealing, galvanizing and galvannealing experiments were performed in the hot dip galvanizing simulator which was accurately controlled both the sample’s temperature and the annealing atmosphere(dew point and H2/N2 ratio). The enrichment and selective oxidation of the alloy elements at steel surface and coating qualities including wettability were characterized using SEM/IA, FIB/TEM, GDS, FT-IR and sessile drop method. The alloying elements(Si/Mn) ratio in IF-HSS and DP-HSS played an important role to predict the compounds apt to oxidize at the surface. That is, when the Si/Mn ratio was below 0.75, IF-HSS exhibited a good wettability leading to a good galvannealed coating quality after annealing at 800℃ for 40s in 15%H2-N2 mixed gas with dew point -60℃. In the Si/Mn ratio above 0.75, In contrast, the wettability and galvannealed coating quality of IF-HSS deteriorated in the Si/Mn ratio above 0.75. In case of DP-HSS, the good coatability and well developed inhibition layer was shown when the Si/Mn ratio was below 0.28. They have relevance to oxides forms by selective oxidation on the steel surface. The oxide particles were dispersed on the steel surface with a surface coverage of below 40% resulted in good wettability and galvannealed coating quality. The oxide particle mainly consisted of Mn2SiO4 with low contact angle in molten zinc. On the other hand, the continuous oxide layer on the steel surface, such as net- and film-type, caused to poor wettability and galvannealed coating quality. The coverage of oxide layer was above 80%, and its chemical species were amorphous SiO2 with high contact angle in molten zinc. The dew points in the annealing atmosphere in IF-HSS and DP-HSS played an important role to predict transition from external to internal selective oxidation. Even if the Si/Mn ratio of IF-HSS and DP-HSS to occur poor wettability and galvannealed coating quality, they exhibited good wettability with molten zinc and galvannealing kinetics after annealing when the dew point of H2-N2 mixed gas was above -20℃ and -30℃, respectively. The wettability and galvannealing kinetics are directly related to the coverage of the external(surface) oxide formed by selective oxidation during annealing. At N2-15%H2 annealing atmosphere, the increase of dew point results in a gradual transition from external to internal selective oxidation. The decrease of surface coverage by selectively oxidized alloying elements with a concurrent increase of their subsurface enrichment in the substrate, showing a larger surface area that was free of oxide particles, contributed to the improved wettability and galvannealing kinetics. On the other hand, the corresponding wettability and galvannealing kinetics were deteriorated with the dew points below -20℃ and -30℃, respectively. The continuous oxide layer of net- and/or film- type was formed on the steel surface, leading to the poor wettability and galvannealing kinetics. It causes a high contact angle between annealed surface and molten zinc, plays an interrupting role in interdiffusion of Zn and Fe during galvannealing process and makes surface defects such as bare spots, craters and alloying delay. Finally, the surface properties of the galvanized high strength steels depends on the Si/Mn ratio and dew points in the annealing atmosphere. Therefore, the sound surface properties of the galvanized high strength steels for automobile can be obtained by controlling these two factors properly.
The effect of minor alloying elements(Si/Mn) ratio in interstitial free-high strength steel(IF-HSS) and dual phase-high strength steel(DP-HSS) containing Si, Mn etc., and dew points in the annealing atmosphere on the surface properties of the galvanized high strength steels for automobile have been studied by galvanizing simulation, with particular attention towards alloying element enrichment, the occurrence of selective oxidation of the surfaces immediately prior to hot dip galvanizing. Annealing, galvanizing and galvannealing experiments were performed in the hot dip galvanizing simulator which was accurately controlled both the sample’s temperature and the annealing atmosphere(dew point and H2/N2 ratio). The enrichment and selective oxidation of the alloy elements at steel surface and coating qualities including wettability were characterized using SEM/IA, FIB/TEM, GDS, FT-IR and sessile drop method. The alloying elements(Si/Mn) ratio in IF-HSS and DP-HSS played an important role to predict the compounds apt to oxidize at the surface. That is, when the Si/Mn ratio was below 0.75, IF-HSS exhibited a good wettability leading to a good galvannealed coating quality after annealing at 800℃ for 40s in 15%H2-N2 mixed gas with dew point -60℃. In the Si/Mn ratio above 0.75, In contrast, the wettability and galvannealed coating quality of IF-HSS deteriorated in the Si/Mn ratio above 0.75. In case of DP-HSS, the good coatability and well developed inhibition layer was shown when the Si/Mn ratio was below 0.28. They have relevance to oxides forms by selective oxidation on the steel surface. The oxide particles were dispersed on the steel surface with a surface coverage of below 40% resulted in good wettability and galvannealed coating quality. The oxide particle mainly consisted of Mn2SiO4 with low contact angle in molten zinc. On the other hand, the continuous oxide layer on the steel surface, such as net- and film-type, caused to poor wettability and galvannealed coating quality. The coverage of oxide layer was above 80%, and its chemical species were amorphous SiO2 with high contact angle in molten zinc. The dew points in the annealing atmosphere in IF-HSS and DP-HSS played an important role to predict transition from external to internal selective oxidation. Even if the Si/Mn ratio of IF-HSS and DP-HSS to occur poor wettability and galvannealed coating quality, they exhibited good wettability with molten zinc and galvannealing kinetics after annealing when the dew point of H2-N2 mixed gas was above -20℃ and -30℃, respectively. The wettability and galvannealing kinetics are directly related to the coverage of the external(surface) oxide formed by selective oxidation during annealing. At N2-15%H2 annealing atmosphere, the increase of dew point results in a gradual transition from external to internal selective oxidation. The decrease of surface coverage by selectively oxidized alloying elements with a concurrent increase of their subsurface enrichment in the substrate, showing a larger surface area that was free of oxide particles, contributed to the improved wettability and galvannealing kinetics. On the other hand, the corresponding wettability and galvannealing kinetics were deteriorated with the dew points below -20℃ and -30℃, respectively. The continuous oxide layer of net- and/or film- type was formed on the steel surface, leading to the poor wettability and galvannealing kinetics. It causes a high contact angle between annealed surface and molten zinc, plays an interrupting role in interdiffusion of Zn and Fe during galvannealing process and makes surface defects such as bare spots, craters and alloying delay. Finally, the surface properties of the galvanized high strength steels depends on the Si/Mn ratio and dew points in the annealing atmosphere. Therefore, the sound surface properties of the galvanized high strength steels for automobile can be obtained by controlling these two factors properly.
주제어
#dew point interstitial free high strength steel dual phase high strength steel hot dip galvanizing
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