Influence of the Dew Point on Coating Quality of GI/GA TRIP steels

MasterThe selective oxidation and the reactive wetting of CMnSi Transformation-Induced Plasticity steels during Galvanizing and Galvannealing were investigated by FE-EPMA and high resolution Transmission Electron Microscopy. During intercritical annealing in a N2+10%H2 gas atmosphere with a dew point in the range of -60˚C to +5 ˚C, a continuous layer of selective oxides was formed on the surface. In a low dew point atmosphere, Mn-rich oxide selectively formed on the surface during heating to 820˚C. Film type of Si oxides and manganese-silicon compound oxides covering the surface were formed during intercritical annealing at 820˚C. In the atmosphere with a dew point of -10˚C, the transition of external to internal oxidation was observed. Regions of external and internal oxidation developed separately. In the high dew point atmospheres, both external and internal oxidation occurred leading to the formation of mixed manganese-silicon oxides started during the heating to 820˚C. The process progressed during intercritical annealing at 820˚C. Simultaneously, small sub-surface ferrite grains formed which had a grain boundary network of manganese-silicon oxides. Annealing in high dew point gas atmosphere resulted in a thinner layer of external oxidation and a greater depth of Mn and Si internal oxidation. The galvanizing simulation was carried out in a Zn bath containing 0.22 mass% Al and the bath temperature was 460 °C. Coarse and discontinuous Fe2Al5-xZnx grains and Fe-Zn intermetallics (ζ and δ) were observed at the steel/coating interface after the hot dip galvanizing of panels annealed in a low dew point atmosphere (-60 ˚C). Poor wetting was observed on panels annealed in a low dew point atmosphere due to the formation of thick film-type oxides on the surface. After annealing in a higher dew point gas atmosphere (-10 °C, and +5 °C), the Fe2Al5-xZnx was continuous and fine grained. No Fe-Zn intermetallics were formed. The small grain size of the inhibition layer was attributed to the nucleation of the Fe2Al5-xZnx grains on fine ferrite subsurface grains and the presence of granular surface oxides. The galvannealing simulation were carried out in a Zn bath containing 0.13 mass% Al and the bath temperature was 460 °C. The galvannealing temperature was in the range of 540-570 ˚C. After the hot dip galvannealing of a panel annealed in a low dew point atmosphere (-60 ˚C), the coating layer was consisted of delta and eta phase crystals. Al and the oxides formed during annealing were present mostly at the steel/coating interface. After the hot dip galvannealing of a panel annealed in higher dew point atmospheres (-10 ˚C and +5 ˚C), the coating layer mainly consisted of delta phase crystals, and thin layer of gamma phase crystals were present at the steel/coating interface. Two types of delta phase crystals were observed: the coarse delta and the fine delta phase crystals. Al was detected mainly at the surface of the coating layer. Oxides were distributed all around the coating layer. The reduction of the surface oxides layer thickness on panels annealed in a high DP gas atmosphere resulted in increased Fe content at the surface of the coating layer. The improved coating quality of the panels annealed in higher dew point gas atmospheres was due to the formation of thinner layer of oxides. A high dew point atmosphere can therefore significantly contribute to the decrease of Zn-coating defects on CMnSi TRIP steels processed in hot dip galvanizing/galvannealing lines

Influence of the Surface Selective Oxidation of Advanced High Strength Steels (AHSS) on the Reactive Wetting in Continuous Hot Dip Galvanizing

DoctorThe formation of an external oxide layer of Mn, Si, and Al during the continuous annealing of advanced high strength steel (AHSS) usually results in the deterioration of the coating quality of the galvanized AHSS. One promising method which was proposed to improve the quality of hot dip Zn coating is to anneal the strip in a high dew point (DP) atmosphere, which results in internal, rather than external, oxidation. This method however requires a modification to the parts of the industrial lines. An alternative method involves the addition of surface active elements such as Sn and Sb to the steel composition. When these elements segregate to the free surface, they suppress the surface oxidation by occupying the oxygen adsorption surface sites. In the present work, the two methods were tested on conventional CMnSi transformation-induced plasticity (TRIP) steels by means of hot dip galvanizing (HDG) simulator tests. The surface oxides formed on the steels during the continuous annealing were analyzed in high resolution by means of transmission electron microscopy (TEM) and 3-dimensional atom probe (3D AP) tomography. The use of the DP control effectively improved the galvanizability of CMnSi TRIP steels processed in hot dip galvanizing/galvannealing lines. This is because the thickness of the surface oxides was effectively reduced by increasing the DP to activate internal selective oxidation. The addition of surface active elements also significantly improved the galvanizability of CMnSi TRIP steels. The addition of these elements changed the morphology of the surface oxides of xMnO•SiO2 from a continuous film to lens-shaped island morphology. Of the five elements, Cr, Ni, Cu, Ti, and Sn, the most significant change was observed when Sn was added. The addition of Sn suppressed the formation of the Mn-rich oxides of MnO and 2MnO•SiO2. The changes in the morphology and chemistry of the surface oxides was due to the surface segregation of Sn, which resulted in a decrease of the oxygen permeability at the surface. The formation of lens-shaped oxides improved the wettability by molten Zn. The improved wetting effect was attributed to an increased area fraction of the surface where the oxide layers were thinner, enabling a direct unhindered reaction between Fe and the Al in the liquid Zn and the formation of the inhibition layer in the initial stages of the hot dipping

Influence of the Annealing Furnace Gas Atmosphere Dew Point on the Galvannealing Reaction on Advanced High Strength Steel

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Influence of Minor Alloying Elements on Selective Oxidation and Reactive Wetting of CMnSi TRIP Steel during Hot Dip Galvanizing

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Quenching and Partitioning (Q&P) Die Quenching

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Application of Quenching and Partitioning (Q&P) Processing to Press Hardening Steels (PHS)

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Influence of Addition of Minor Alloying Elements on the Selective Oxidation and Reactive Wetting during Hot Dip Galvanizing of CMnSi TRIP Steel

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