GALVANNEALING OF DUAL PHASE STEELS

Abstract

<p>The high strength and ductility of dual phase (DP) steels makes them ideal for use in the automotive industry. However, to be used in automotive exposed parts galvanizing (GI) and galvannealing (GA) processes are essential to provide corrosion protection. Galvannealing of dual phase steels has three major steps: i) heat treatment of the steel strip to obtain a suitable substrate microstructure and reduce iron oxides at the substrate surface ii) dipping of the steel strip in the zinc bath to obtain a soft and ductile metallic zinc coating on the steel and iii) heat treatment of the coated substrate in the galvannealing furnace after removal from the zinc bath to form an Fe-Zn intermetallic coating on the steel.</p> <p>The major challenges in galvannealing of dual phase steels are selective oxidation of the alloying elements used in DP steels such as Mn which may result in poor galvannealed coatings, and galvannealing time and temperature that can affect the microstructure and formation kinetics of galvannealed coating. Both of these issues have been investigated in this research using three industrial steel substrates: EDDS (Extra Deep Drawing Steel), CMn (Carbon Manganese) and DP590 (Dual Phase).The concentration of carbon, manganese and some other alloying elements was different in these substrates.</p> <p>The effect of process atmosphere oxygen partial pressure on oxidation was determined for all experimental steels at dew point (dp) -30°C using a N₂-5%H₂ process atmosphere. The steel chemistry and oxygen partial pressure of the process atmosphere affected oxide thickness and morphology. For all alloys the lowest oxygen partial pressure process atmosphere resulted in the highest concentration and thickest oxide layer of Mn at the surface of dual phase steel (DP590). Also, the lowest segregation of Mn and thinnest oxide layer of Mn at the surface was obtained for the EDDS steel. The predominant oxide morphology observed at the surface of the DP590 steel comprised large oxide nodules or thick oxide films with irregular shaped/faceted nodules whereas the other two steels had an oxide morphology that generally comprised spherical cap shaped nodules at grain boundaries.</p> <p>Four galvannealing times (10, 20, 30 and 40 s) and three galvannealing temperatures (480, 500 and 520 °C) were used to evaluate the effects of GA time/temperature on the microstructural evolution and formation kinetics of coating as a function of substrate Mn content. By increasing the galvannealing time and temperature, it was observed that for all steels, the Fe-Zn growth rate (alloying rate), thickness of gamma layer (Γ-Fe₃Zn₁₀) and iron content of the galvannealed coating were increased. It was concluded that galvannealing kinetics of DP and CMn steels at 480°C are faster than those of the EDDS steel. However, the galvannealing kinetics of DP and CMn steels at 500 and 520°C were relatively similar to each other and insignificantly different than those of EDDS. Accelerated galvannealing kinetics of higher Mn containing steels in this research, i.e. DP and CMn, could be ascribed to the presence of thicker oxide film/larger oxide particles at the surface that may have been reduced by aluminothermic reduction and accelerated inhibition layer breakdown. Considering the alloying rate and chemistry of the GA coating, it was found that 500 and 520 °C are not suitable industrial galvannealing temperatures for experimental steels in this research.</p>