• PDF / 404,035 Bytes
• 8 Pages / 612 x 792 pts (letter) Page_size
• 19 Downloads / 250 Views

DOWNLOAD

REPORT

---

I. INTRODUCTION

CHROMIUM stainless steels have widely been used for corrosion resisting materials at room or high temperatures as well as construction materials. However, the oxide inclusions such as spinel (MgAl2O4) probably deteriorate corrosion resistance and cause the surface defects and cracks during deep drawing or spinning manufacturing. Hence, some researchers have investigated the formation behavior of spinel in stainless steel melts for about the last 10 years. Harkness and Dyson reported that the spinel, which has a stable fcc structure with high melting point, would readily form in stainless steel melts and this inclusion was not effectively suppressed by calcium addition from their plant data.[1] Kim et al. reported that the spinel phase could be crystallized in the suspended CaO-SiO2-MgO-Al2O3(-TiO2) inclusion matrix as the temperature of molten steel decreased from about 1923 K at AOD converter to about 1723 K at continuous casting mold.[2] From the analysis of plant data, they proposed that the aluminum content in steel melt and MgO content in AOD slag should be lowered to suppress the formation of spinel. The crystallization of spinel in the liquid CaO-SiO2-MgO-Al2O3 inclusion matrix, with Al2O3 content greater than about 20 mass pct, has also recently been reported by Ehara et al.[3] However, in these works, the effect of slag compositions on the formation mechanism of spinel in molten steel has not been clearly understood.

Nishi and Shinme observed that the content of MgO in the Al2O3-based inclusions, readily formed by aluminum deoxidation, increased with increasing basicity (
(pct CaO)/(pct SiO2)) of the CaO-SiO2-MgO-Al2O3 slags.[4] They proposed that the reduction of MgO in the slags by aluminum in molten steel at the slag/metal interface would provide small amounts of magnesium, which could react with Al2O3 inclusions, albeit less than 1 mass ppm magnesium. The similar mechanisms were suggested by Okuyama et al.[5] and Todoroki and Mizuno.[6] However, because the content of aluminum in molten steel was from about 0.01 to 1 mass pct in these studies, the initial inclusions were Al2O3, regardless of slag compositions. Furthermore, although the CaO-SiO2-MgO-Al2O3 (-CaF2) slags were reacted with the steel melts containing aluminum, only the reduction of MgO was considered. However, the SiO2 could also be reduced by such a large content of aluminum, which was not taken into account. Therefore, in the present study, the equilibrium reaction between the Fe-16Cr melts and the CaO-Al2O3-MgO slags was investigated to understand the formation behavior of the MgO-Al2O3 spinel-type inclusions in ferritic stainless steel under condition of aluminum content less than about 0.02 mass pct. The composition of the slags was chosen as the systems saturated by the solid phase for definite thermodynamic conditions.

II. EXPERIMENTAL JOO HYUN PARK, Senior Researcher, and DONG SIK KIM, Senior Researcher, are with the Stainless Steel Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Con