Effect of dew point on galvanizability of Si-Al added transformation induced plasticity steels

• Main Authors: Yi-pei Liu, 劉宜佩
• Other Authors: Liu-wen Chang
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/75499794831670859244

碩士 === 國立中山大學 === 材料與光電科學學系研究所 === 103 === Two Mn-Si-Al added transformation-induced plasticity steels, TRIP A and TRIP B, were studied to clarify the effect of dew point during annealing on the selective oxidation and galvanizability in hot-dipping. Three dew point of -70 oC, -30 oC and 0 oC were used. Samples were analyzed by scanning electron microscope (SEM), for the morphology and distribution of oxides and Fe-Al inhibition crystals. The chemical characteristics of the oxides were analyzed by X-ray photoelectron spectroscopy (XPS). The crystallivity, morphology and composition of the oxides were studied by transmission electron microscope (TEM). Results indicated that the surface of TRIP A was covered by MnO and complex oxide containing Mn, Al and Si after it was annealed at a high dew point of 0 oC. The quantity of MnO increased with increasing the annealing temperature from 650 oC to 700 oC, but decreased dramatically at 800 oC. Accordingly, the surface oxides were composed of roughly equal quantities of complex oxide, enriched with Si and Al, and MnO. Decreasing the dew point to -70 oC, the surface oxides are mainly complex oxides with a small quantity of MnO whose fraction decreased with increasing the annealing temperature. At 800 oC, a large amount of SiO2 was also formed on the surface. TEM analyses showed that the complex oxide is amorphous with nano-sized -Al2O3 distributed throughout the amorphous oxide. The Mn and Si contents in the complex oxide decreased with decreasing the dew point. The surface oxides form on the surface of TRIP B annealed at a high dew point of 0 oC were mainly MnO associated with a small amount of complex oxide. At dew point of -70 oC, the surface oxides were still MnO dominated as the steels were annealed at 650 and 700 oC. However, The complex became the major oxide formed on the surface at 800 oC. In addition, the Mn conten in the complex oxide also decreased dramatically. The complex oxide remained as amorphous with nano-sized -Al2O3 precipitated in it. The TRIP A steel possessed a good galvanizability as hop-dipped in a 0.16 wt% Al bath after annealed at 800 oC at 0 oC dew point. Some bared spots were observed in the coating as the steel was annealed at -30 oC dew point, whereas the sample was not wetted to Zn after annealed at a dew point of -70 oC. For TRIP B, bared spots could not be avoided even whae the steel was annealed at a dew point of 0 oC. The coatability became worse as the annealing dew point decreased. According to the above results, it is concluded that a layer of 50 nm thick MnO formed on the steel surface is not harmful for galvanizing in a 0.16 wt% bath. However, a layer of amorphous complex oxide of 30 nm in thickness can totally inhibit the formation of Fe-Al compound. The Zn is not wetted to the steel surface accordingly. Limited coatability can be obtained for steels covered by a mixed layer of MnO and complex oxide. The coatability gets worse as the area fraction of the complex oxide increases.