The Impact of Selective Oxidation on the Phase Transformation in the Sub‐Surface of Advanced High Strength Steels

Abstract

The present paper contributes in the discussion of heterogeneous gas/metal reactions by discussing the influence of the dew point (dp) during intercritical annealing on the sub-surface constitution. Annealing trials with different Advanced High Strength Steels (AHSS) were carried out and the element distribution within the sub-surface was analysed by glow discharge optical emission spectrometer (GD-OES). For modelling purpose the gained element distribution data were adjusted in a way that the selective oxidation products were considered within the sub-surface element profiles. Several transformation temperatures along the depth profiles according to the adjusted GD-OES data have been computed for the dp À30 8C and þ5 8C. In all cases the according models have been implemented in Matlab. The thermodynamic data have been obtained via the Matlab-ThermoCalc interface. Empirical equations have been applied for the determination of the bainite and martensite start temperatures. It is shown that the sub-surface constitution and the transformation temperature differ significantly. In most cases the change of the transformation temperature within the surface and sub-surface reaches up to 100 8C compared to the bulk. For a Dual-Phase (DP) Steel with C(0.15%)Mn(1.7%)Al(1.7%)Cr(0.5%), the change of the A e1 -temperature is with 300 8C significantly higher. However one has to keep in mind that the 2-phase composition can not be assumed to be constant during intercritical annealing. It is therefore concluded that for the purpose of simulating the selective oxidation processes during intercritical annealing of AHSS the continuous change of the sub-surface constitution must be incorporated in the future work.

High Temperature Oxidation of Indefinite Chill Roll Material Under Dry and Humid Atmospheres

“…An optical microscope (OM) was observed using a DM6000 optical microscope. Additionally, the morphology and the chemical composition of the oxidation products was determined with scanning electron microscopy (SEM) using a JEOL JSM 6490 and filled emission JEOL JSM‐7001F SEM . The constitutions of the oxide scales were identified by X‐ray diffraction (XRD) on a GBC MMA diffractometer.…”

“…Additionally, the morphology and the chemical composition of the oxidation products was determined with scanning electron microscopy (SEM) using a JEOL JSM 6490 and filled emission JEOL JSM-7001F SEM. [19,20] The constitutions of the oxide scales were identified by X-ray diffraction (XRD) on a GBC MMA diffractometer. The cross-section segments have been cut from each sample, mounted in Bakelite, and polished down to 0.25 mm.…”

Influência Do Ponto De Orvalho Na Oxidação Seletiva Dos Ahss

“…Amostras dos materiais processados foram obtidas para a realização deanálises (metalográficas, MEV, EDS) para efeito de comparaçãocom processo inicial (T P )avaliando-se a morfologia e composição química tanto da camada intermetálica quanto a oxidação seletiva superficial quando presentes. As primeiras amostras analisadas foram para ponto de orvalho T P +15, em que se espera encontrar uma estrutura similar às apresentadas pelo grupo de Bellhouse e McDermid [6,7] que utiliza valores similares e mesmos teores de H2que possui influência significativa sobre o material [8,9]. Assim, a Figura 04apresenta a constituição uniforme e homogênea da camada intermetálica, na qual a morfologia observada nessa região corresponde ao formato típico de "grãos de arroz", característica da camada intermetálica Fe 2 Al 5 Zn x (Figura 04c), a qual propicia melhor a aderência entre o aço e o revestimento.…”

Local Equilibrium and Nodule Formation on Al‐TRIP‐Aided Steel Surfaces During Intercritical Annealing

“…The surface reactivity of AHSS can be increased by introducing water vapor into the annealing furnace or by using pre‐oxidation 32–35. But the improvement is limited; as at high dp conditions, decarburization is significantly encouraged 36–40. On the other hand, enhancing the surface reactivity of Si containing AHSS is limited, due to the high oxygen affinity of Si.…”

“…Nevertheless, the amount of absorbed oxygen is dependent on the adsorbed amount of the oxygen at the steel strip surface. Due to this, not only the oxygen adsorption of the annealing furnace gases equilibrium should be taking into account as a source of oxygen for the selective oxidation process, but more importantly, the impacted of the adsorbed water vapor has to be considered, as following equilibrium reaction takes place at the steel surface as well40: where K f(ads) is the reaction constant (isotherm), p ads (H 2 O) the partial pressure of water vapor adsorbed at surface [bar], p ads (O) the partial pressure of oxygen adsorbed at surface [bar], and p ads (H) is the partial pressure of hydrogen adsorbed at surface [bar].…”