Internal and External Oxidation of Manganese in Advanced High Strength Steels

Abstract

Advanced high strength steels (AHSS) have been used extensively in the automotive industries. The main characteristic of these steels is combination of high strength and enhanced formability that makes them very attractive for automotive application. However, the major drawback of these steels is their poor corrosion resistance. Hence, Hot-dip galvanizing can be used to assure the corrosion resistance of AHSS. Mn is one of the most common alloying elements in these steels. Its selective oxidation during annealing prior to galvanizing is of concern, since MnO at the steel surface deteriorates the wettability between steel and the molten zinc. In Chapter 1 a general description of advanced high strength steels is presented. The environmental aspects of conventional steels, advanced high strength steels and other alternative materials are compared. The theoretical background of internal and external oxidation modes are dealt with in Chapter 2. The kinetics of internal oxidation, solubility product, diffusion of oxygen, nucleation and supersaturation are discussed. In Chapter 3, transition from internal to external oxidation of three Mn-steel alloys is described. Mn-steel alloys containing 1.7, 3.5 and 7.0 wt% Mn were oxidized at 950 °C in the dew point range of -45 to +10 °C in N2 plus 5 vol.% H2 gas mixture. The experimental procedure and characterization techniques are presented. Based on the observed selective oxidation behaviour of Mn in the steels MnO can be considered as a low solubility product oxide. The size of the internal oxide precipitates depends on the initial Mn concentration and increases with oxidation time. Volume fraction of internal oxide precipitates is constant within the internal oxidation zone (IOZ) and decreases to zero beyond the internal oxidation front. The critical volume fraction of internal oxide particles corresponding to the transition from internal to external oxidation is determined. The value of this critical volume fraction equals 0.20 ± 0.02, which is much smaller than the value that is generally adopted, viz.: 0.3. The kinetics of internal oxidation of the three Mn-steel alloys is addressed in Chapter 4. For these alloys, the kinetics of internal oxidation obeys parabolic growth rate law, i.e. diffusion-controlled. Using the observed internal oxidation kinetics, the diffusion coefficient of oxygen and manganese in ?-Fe are determined. The corresponding values are 3.35×10-7 and 4.14×10-12 cm2/s at 950 °C, respectively. The solubility product of MnO in ?-Fe is estimated to be (7.66 ± 0.18)×10-9 mole fraction2 at 950 °C. A numerical model was developed for internal oxidation of binary alloys. The developed model successfully predicts the concentration depth profile of precipitated oxygen using the above-mentioned parameters. Furthermore, phase transformation from austenite to ferrite may occur as a result of Mn depletion from the matrix due to internal oxidation. For the oxidation of 1.7 wt% Mn alloy at 850 °C, the internal oxidation was simulated with the developed model. If the phase transformation of the steel matrix upon oxidation is included, the predicted depth of internal oxidation zone is in agreement with experimental observations. The chemical composition of oxides formed during oxidation Mn-steels at 950 °C in a N2 plus 5 vol.% H2 gas mixture with different dew points was determined using X-ray diffractometry, X-ray microanalysis and X-ray photoelectron spectroscopy. The results are presented in Chapter 5. At the lowest dew point, i.e. -45 °C, MnO was formed at the surface. However, at higher dew points, a mixed oxide composed of FeO and MnO was formed. In agreement with thermodynamic calculations, the concentration of FeO in the mixed oxide increases with increasing dew point (oxygen partial pressure). Remarkably, oxidation of Fe occurs when Mn is present in the steel, while in the absence of Mn, i.e. pure iron, no oxidation happens at the dew points up to +25 °C. The oxide composition is independent of the Mn concentration for the oxidation conditions considered in this study. The formed oxides have a metal-deficit non-stoichiometry, with an atomic defect fraction of 0.19 ± 0.03. Hence, the defect concentration is independent of dew point for the conditions considered in this thesis. This was also confirmed with the oxygen Auger parameter. The oxygen Auger parameter offers information about the local chemical environment around the oxygen atoms. Although the oxide composition changes with dew point, the oxygen Auger parameter remains constant.