A Study On Selective Oxidation Behaviors Of DP780Dual Phase Steels During Hot-dip Galvanizing Process

The light autobody is being paid more and more attention since it is an effective way to reduce oil consumption. The light autobody is utilized by applications of advanced high strength steels, which can reduce sheet gauges and maintain autobody safety at the same time. Dual phase steels account for74%of advanced high strength steels used in the light autobody.When the dual phase steels are used in autobody, they have to be coated with a zinc layer by hot-dip galvanizing process to enhance the anti-corrosion capability of autobodys. However, dual phase steels readily give rise to surface selective oxides during annealing prior to galvanizing, since they are alloyed certain contents of alloying elements. The surface selective oxides have bad wetting behaviors with zinc bath, and can cause severe coating defects such as bare spots and bad coating adherence. Therefore, understanding selective oxidation of alloying elements and formation of selective oxides are essential and fundamental to improve zinc coating quality during galvanizing dual phase steels. This dissertation aims to investigate the effects of process parameters on selective oxidation of alloying elements and further understand interfacial reactions of oxides with effective Al in zinc bath, which occur in substrate/zinc interface. Some creative achievements concerning hot-dip galvanizing dual phase steels have been made.The influences of annealing duration and annealing temperature on selective oxidation have been investigated. The results show that as annealing duration prolonged and annealing temperature raised the density and size of surface oxides increase. And some coarse intermetallic Fe-Al grains emerge in the inhibition layer. XPS characterization shows that the variations of annealing temperature and duration do not change chemical natures of selective oxides.The segregation to steel surface is quite different for various alloying elements. Mn has a strong tendency to diffuse to steel surface, resulting in a sever segregation on steel surface so that MnO is the main surface oxide. Cr has a much less tendency of segregation on steel surface, and the amounts of Cr2O3are much less than MnO. Mo can hardly segregate on steel surface and virtually no Mo oxides could be traced on steel surface.By means of adjusting hydrogen content and dew point, the gas-metal reaction occurred in annealing process is examined. The results show that as hydrogen contents increased from5%to20%, the density of surface oxides increase. Moreover, the variations of hydrogen contents do not change chemical natures of selective oxides. When dew point rises from-50℃to-15℃, internal oxidation of alloying elements takes place, and consequently the mass of external oxides decrease. With the concept of preoxidation-reduction, annealed dual phase steels with large proportion of bare surface are obtained when the dew point is set as0℃/-70℃.The aluminothermic reaction of MnO with effective Al is investigated. And the maximum volume of MnO that is able to be reduced by effective Al in the limited galvanizing time of3s is obtained, which is equal to131.6nm. The growth mechanism of Fe-Al grains is discussed. Furthermore, a correlation between the size of MnO and the formation of coarse Fe-Al grains is given. A size effect of oxides on nucleation of Fe-Al intermetallic grains is proposed.