Investigation On The Surface Selec-Tive Oxidation And Galvanisability Of Advanced High Strength Steels

Firstly in this work, the surface selective oxidation of several advanced highstrength steels and model alloys was fundamentally investigated in the context of thecontinuous hot-dip galvanizing. Then the impact of selective oxidation on the gal-vanisability of sheet steel was examined. Finally, this work explored the influence ofthe galvanizing thermal route on the mechanical properties of three dual phase steelsand one TRIP-aided steel.By establishing a local thermodynamic equilibrium region, the influence of steelchemisty and annealing atmosphere on the selective oxidation was investigated bythermodynamics. It was found that the selective oxidation of element Al, Cr, Mn, Pand Si occurred in the galvanizing atmosphere is thermodynamical favorable andunavoidable. The type of oxides present on the steel surface depends on the alloy el-ements and their content in the matrix. Element Si, Al and Cr can react with Mn aswell as O to form mixed oxides, which follows the so-called Mn-Si, Al-Mn or Mn-Crrules. Element Mo doesn’t take part in the selective oxidation due to its similarchemical stability with Fe. However, combining the experiment results of this work,it is nessaccery to point out that we must also consider the diffusion kinetics of oxy-gen and alloy elements through the local equilibrium region.The selective oxidation behavior of four model alloys at two dew points,-30℃and+10℃, were studied by XPS and FESEM-EDS. The results show that the chem-ical composition of the alloy matrix, the initial status of the model alloy and the an-nealing dew point influenced the oxidation behavior. The oxidation of Mn and Sifollows the Mn-Si rule. In the annealing interval of120s, Si up to1.5wt.%in the ma-trix contributed a lot to the surface oxidation, and Cr failed to combine Mn to formthe Mn-Cr-O mixed oxide. Moreover, Cr segregated to the surface after annealed atboth dew points. As the dew point increases, the morphology of the surface oxidechanged from film/network shape to granular/island shape, and the surface coverage of oxide tends to decrease. Since the cast alloy has large grain size and small amountof grain bourdaries, the amout of surface segregation is low, and is not influenced bythe dew point. While for hot rolled samples, the amount of surface segregation ishigher than cast alloy, and is influenced by the dew point. However, the initial statushas no influence on the type of oxides on the surface.Dew point significantly influenced the surface segregation and oxidation behav-ior of alloy elements in TRIP steel. The increase of the dew point leads to the internaloxidation of Al and Cr. When at low dew point-30℃, P does not take part in the sur-face segregation and selective oxidation, while increasing the dew point to+10℃leads to the surface segregation of this element.Meanwhile, increase the dew point change the oxide from film shape to island orgranular shape, decreased the surface coverage. The selective oxidation behavior onthe surface of DP steels is similar with that of TRIP steel. The addition of Cr and Moto DP steel changed the segregation and oxidation behavior of Mn and Si. It shouldbe noted that due to the consumption of alloy elements by internal oxidation and de-carburization, increasing the dew point to+10℃led to the formation of a ferriteband in the subsurface area in TRIP steel, also made the subsurface of DP steel easierto be etched by1vol.%nital solution. Furthermore, the addition of Cr and Mo to DPsteel changed the morphology of the internal oxide.The wetting experiment results on the oxidized surface of model alloys show thatthe morphology of the surface oxide determined the wetting property. Surface withsmall oxide particles is well wetted by zinc with a fast wetting kinetics, while surfacewith film-shape oxide or large size oxide particle is not wetted by the zinc with aslow wetting kinetics. The high content of Si up to1.5wt.%significantly deterioratethe wetting property of the alloy surface with zinc. The result of the reactive interfaceanalysis shows that all the bare spot are located at areas with thick oxide, such asgrain boundary. Although there are some intermetallic compounds on the interface,these compounds are not compact and with considerabe conent of oxygen. XPS anal- ysis on the interface shows that the metallic iron and Mn were present due to the re-duction effect of Al from the zinc drop.The galvanisability of the industrial steels is also related to the morphology ofthe surface oxide. Film shape oxide deteriorated the galvanisability significantly,while small oxide particle showed less influence on the galvanisability. Variation ofthe dew point can change the morphology of the oxide and improve the galvanisabil-ity; however, this improvement is very limited. The effective Al in from the zinc bathcan reduce some oxide on the steel surface; however, the local depletion of Al influ-enced the size and compactness of the intermetallic compounds. Due to the thick ox-ide present on the grain boundary or some spot within steel grains, the Al can not re-duce all the oxides in a short dipping time. Therefore, those spots can not be wettedby zinc and become the bare spot after galvanizing.The present work also found that the thermal route of continuous hot-dip galva-nizing has an impact on the mechanical properties of the industrial steels. For TRIPsteel, it is very important to employ appropriate parameters of heat treatment to pro-duce hot-dip galvanized products with promising mechanical properties. For DP steel,an increase of the cooling rate after inter-critical annealing and a decrease of dippingtime is benefit for the mechanical properties. Further, the addition of Cr and Mo canincrease the compactibility of DP steel with the continuous hot-dip galvanizing.