Formation Mechanism of Spinel-Type Inclusions in High-Alloyed Stainless Steel Melts
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NTRODUCTION
OXIDE inclusions, especially hard aluminates, are known to be harmful to the mechanical properties of the steel welds as well as to the steel material itself.[1,2] Mukhopadhyay et al.[3] reported that the tensile deformation behavior of notched 18 mass pct Cr-10 mass pct Ni austenitic stainless steel was dependent on the inclusion decohesion. Giordani et al.[4] proposed that nonmetallic inclusions containing aluminates were detrimental to the initiation of the corrosion-fatigue cracks in high-alloyed (23 mass pct Cr-11 mass pct Ni) stainless steels. Magnesium aluminate (MgAl2O4, spinel) inclusions are deleterious not only to the surface brightness but also to the press formability of the stainless steel products. These spinel-type inclusions originate from the inclusions in continuous cast slabs, as shown in Figure 1, indicating that the spinel phase could crystallize in the calcium aluminosilicate inclusion matrix during transfer of molten steel from an argon oxygen decarburization (AOD) converter to the continuous casting (CC) machine via tundish and mold in the JOO HYUN PARK, Senior Researcher, is with the Stainless Steel Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Contact e-mail: [email protected] Manuscript submitted January 3, 2007. Article published online July 7, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
conventional stainless steelmaking processes. Hence, some industrial researchers have investigated the formation behavior of spinel inclusions based on laboratory experiments or plant trials, which are simply reviewed as follows. Harkness and Dyson reported that the spinel, which has a stable face-centered cubic structure with high melting point, would readily form in the austenitic stainless steel melt and that this inclusion is not effectively suppressed by calcium treatment.[5] This was in accordance with the description of Kor.[6] In the results of Kim et al.,[7] the spinel phase could crystallize in the suspended CaO-SiO2-MgO-Al2O3 (-TiO2) inclusion matrix as the temperature of the stainless steel melts is decreased from about 1923 to 1723 K during the transfer of steel melts from the AOD converter to the CC mold. From the analysis of plant data, they proposed that the aluminum content in steel melts and the MgO content in AOD slags should be lowered to suppress the formation of spinel crystals. The similar results were presented by Hojo et al.[8] with the conclusion that a decrease in the content of MgO and Al2O3 in AOD slags could be effective for preventing the crystallization of spinel in the inclusion droplets. More recently, Ehara et al.[9] observed that the spinel phase was crystallized in the CaO-SiO2-MgO-Al2O3 liquid melts at alumina content greater than about 20 mass pct with a cooling rate of VOLUME 38B, AUGUST 2007—657
Table I.
Experimental Composition of the Steel Melts Metal Composition [mass pct]
No.
Silicon
Aluminum
1 2 3 4 5 6 7 8 9 10 11
3.0 3.0 3.4 3.2 2.1 3.0 2.9 3.1 3.1 3.0 3.1
0.0190 0.0136 0.0158 0.0172 0.0044 0.0130 0
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