Effect of liquid phase on scale formation during high-temperature oxidation of AlSi-transformation-induced plasticity steel surfaces

Abstract

The oxidation kinetics, and the structural evolution of the resulting surface scale, of cast transformationinduced plasticity (TRIP) steel (0.97 wt pct Al and 1.11 wt pct Si) has been investigated in the temperature range of 850 °C to 1250 °C under atmospheres with oxygen partial pressures close to 0.2 atm. Direct visualization using a high-temperature confocal scanning laser microscope (CSLM) showed that at 1050 °C and higher temperatures, a liquid oxide phase formed beneath the surface, penetrating and liquefying the scale that had formed initially. After a period of time, which was dependent on temperature, the liquid became fully crystallized. A microprobe analysis of the steel/scale interface indicated an Al 2 O 3 -SiO 2 -FeO n composition in the liquid oxide. This phase formed a network that penetrated the scale. The rest of the outer scale consisted primarily of Fe 2 O 3 , while Al-Si-rich oxides were observed close to the metal/scale interface. Thermogravimetric analysis indicated a parabolic growth rate below 1000 °C and a linear growth rate at 1000 °C. At higher temperatures, a parabolic rate dominated once again. The scale thickness appears to be limited by the time period during which the liquid oxide could contribute to rapid mass transfer, which resulted in the observed linear oxidation rate. As the upper temperature limit of the linear oxidation region is reached, the liquid oxide becomes enriched with FeO n , decreasing the stability of the liquid phase. This leads to crystallization of solid Fe oxides at the surface or the formation of appreciable amounts of Al-and Si-rich oxides at the interface. These processes block access of the liquid oxide to the steel.

Effect of TiO₂ Addition on Crystallization Characteristics of CaO-Al₂O₃-based Mould Fluxes for High Al Steel Casting

“…high Al-TRIP and TWIP steel) has been received increasingly attention due to their excellent combination of high strength and superior formability. 1) When casting high aluminum steel, due to the reaction between aluminum in steel and silica in the traditional limesilica based mould flux as shown in Eq. (1), it is a great challenge to achieve good casting performance.…”

The Influence of Al₂O₃/SiO₂ Ratio on the Viscosity of Mold Fluxes

“…Transformation-induced plasticity assisted steels (TRIP steels) are one of the most ideal candidates for lightweight automotive applications because of their strength to weight ratio and excellent deformability. [1][2][3] In order to obtain these properties, TRIP steels have conventionally contained C, Mn and Si, where C and Mn are used to stabilize austenite and Si is used as a solid solution strengthener and to suppress iron carbide formation. 4) In view of the detrimental effect of Si-based oxides that form on the surface, on the coating process, the replacement of Si by Al has been explored [5][6][7] and the experimental results indicate that Al has similar effect to Si in influencing the iron carbide precipitation and stabilization of austenite.…”

“…One such property is the viscosity of the molten slag film that infiltrates the mold/strand gap and has to be tailored such that adequate lubrication is provided. In order to ensure adequate lubrication, the viscosity (h) and the casting speed (v), should satisfy the following equation, 12) h (poise)ϫv (m/min)ϭ1.0-3.5 .................. (2) The viscosity of a mold-slag is determined by (i) the degree of polymerization and (ii) the size and shape of the polymeric units. Generally, industrial slags and fluxes contain: SiO 2 , Me x O (metal oxides) and depending on the slag additional compounds like Al 2 O 3 , CaF 2 , Na 2 O and P 2 O 5 .…”

Crystallization Behaviors of Slags through a Heat Flux Simulator

“…In recent years, advanced high strength steels (AHSS) have attracted much attention to be used as car bodies due to its excellent combination of high strength and ductility, [13][14][15] which necessitate the development of AHSS steels for continuous production in steel plants. However, there are several problems that need to be overcome and some are related to the high amount of reactive elements in these alloys.…”