Effect of Argon Stirring on Inclusion Flotation Using a Porous Plug

Transcription

1 ISC Research Symposium Spring 21 Effect of Argon Stirring on Inclusion Flotation Using a Porous Plug Advisor: Dr. K. Peaslee Andrew O Loughlin April 12, 21

2 Abstract: This report documents the use of argon stirring with a porous plug in 1lb ladles to improve the flotation of inclusions before casting. The effect of the rate of stirring was investigated to determine the effect on heat los in the ladle (energy considerations), steel cleanliness and gas content of nitrogen and oxygen. The trial did not show improvements in cast properties over the standard ladle practice, but in fact the application reduced steel cleanliness. The effectiveness of the method was limited by the ability to stir the melt without breaking the protective slag surface layer. Introduction: Inclusions predominately entrapped oxides and sulfides in steel that serve as the primary sites for void nucleation and growth of crack propagation. To improve the mechanical properties of steel, ladle treatments are applied to improve inclusion flotation through deoxidization reactions (Ca treatment) or stirring the melt with inert gas. Argon stirring with a porous plug is a common practice in industrial continuous casting operations and larger foundry ladle operations. Stirring with argon promotes the flotation of inclusions and dissolved gases (H, O and N), and aid in the homogenization of the melt chemistry and temperature[1]. The use of porous plugs in large ladles (greater than 4 tons) offer several advantages over lance stirring. These include: better stirring at the bottom of the ladle, the protective slag layer at the surface of the melt is not broken (for a nonviolent stir rates), and more efficient operational controls [2]. In a previous study conducted by Vintee Singh, the practice of argon lance stirring and Ca wire treatment were investigated in 1lb ladles. The trial showed a slight improvement in inclusion flotation using the lance stirring, but the Ca wire treatment provided a significant improvement in steel cleanliness [3]. Experimental Procedure: The Ar stir rate was studied for seven ladles of medium carbon steel (Table I). For each ladle, steel chemical samples were collected before stirring, after stirring, part way through the pour and the mold. The three ladle samples were collected using submerged chemistry samplers. Keel bar molds were cast for each to provide material for Charpy impact tests. The temperature of the melt was monitored at each process step to monitor the rate of heat loss in the melt. The Ar flow rate was controlled to provide three different levels of stirring: a short violent stir (2.9 CFM for 2 min), a gentile stir (1. CFM for 4 and 4.5 min) and a medium stir (1.9 CFM for 1, 2, and twice at 3 min). Table I: Average Chemistry of the Steel Castings. C Mn P S Si Cu Ni Mo Cr V Al Ti Fe.2 to.45 to.1 to.18 to.43 to.75 to.14 to.31 to.243 to.8 to.17 to.52 to Bal

3 The oxygen and nitrogen gas content was measured using a Leco TC-5. To ensure accurate values, three.7 to 1.g steel sections were tested per sample. To measures the oxygen and nitrogen, the machine melts the steel coupons in an electrode impulse furnace and runs solid state infrared and thermal conductivity measurements on the molten pool. The steel cleanliness was evaluated using an ASPEX analytical SEM particle analyzer to quantify the inclusion content. Over 1 inclusions per sample were dimensionally analyzed and compositionally analyzed using EDX 1. To compare the inclusion analysis to the previous Ca-Wire injection study, the same inclusion classification rules were used (according to EDX composition). The keel bars were normalized at 165 o F for 1.5 hrs and air cooled. Three V-Notch Charpy impact bars were machined for each condition and were tested at -4 o F. Results and Discussion: A complete overview of the Ar stir trials is provided in Table II. The change in inclusion and gas content throughout the processing steps contains significant scatter, but there is a steady trend of increasing the fraction of inclusion coverage with processing steps. This is contrary to the previous study conducted by Vintee Singh, where the inclusion content declined with ladle hold time and Ar Stirring [3]. Table II: Overview of the porous plug Ar-stir trial ladle treatments. Heat and Ladle Number Average Stir Time (min.) Stir Rate (CFM) Temperature ( o F) Inclusion Count (#/mm 2 ) Inclusion Coverage (µm 2 /mm 2 ) Oxygen (ppm) Before Stir Nitrogen (ppm) Temperature ( o F) Inclusion Count (#/mm 2 ) Inclusion Coverage (µm 2 /mm 2 ) Oxygen (ppm) After Stir Nitrogen (ppm) Inclusion Count (#/mm 2 ) Inclusion Coverage (µm 2 /mm 2 ) Oxygen (ppm) Nitrogen (ppm) Final Casting Charpy Impact Energy (Ft-lbs) 6.3 ± ± ± ± ± ± ± ± 1.4 Appendix A provides a comparison of the inclusion content, O and N levels and the impact toughness measurements from the keel bar mold samples for all of the porous plug trails and the inclusion and toughness measurements from the previous trial (Ar stirring with a lance and Ca-wire injection). The primary difference between the inclusion content in the two studies can be attributed to elevated levels of

4 MnS and TiO detected in the porous plug trials (Figure 1 b and Figure 2). Two measurements of the inclusion content were reported. The inclusion count (number of inclusions per mm 2 ) and the inclusion coverage (area fraction of inclusions over the area of metal scanned). The inclusion coverage is a better indicator of steel cleanliness, because is accounts for the size of inclusions. The inclusion content and inclusion coverage data for the porous plug showed significant scatter and did not follow the expected trend of decreasing fraction area covered with increasing stir time (Figure 1 a & b). The gas content in the final castings was stable and did not show any trend with increasing stir time or stir rate (Figure 1c). The oxygen values were low (~7 to 9 ppm) and did not vary with differences in oxides inclusions. In comparison, the nitrogen values were large (16-19 ppm), but if is not uncommon for foundries to contain large nitrogen content. When compared to the previous trial data shown in Figure 2, the fraction of area covered by inclusions in the porous plug samples was substantially greater that the untreated standard practice trial (.1 to.3 compared to.1 area of inclusions over the surface area scanned). The Charpy impact testing observed very low energy absorbed values ranging from 5 to 9 ft-lbs (Figure 3a). This was comparable to the lower limit of mechanical properties observed in the Ca-wire trial (Figure 3b). Ar stirring with the porous plug was not an appropriate ladle treatment method to improve the mechanical properties of the 1lb ladle castings. The negative impact of Ar stirring on inclusion content indicates the protective slag layer was disrupted even at low stir rates. The 1 pound ladles are at a disadvantage to larger ladles (greater than 4 ton), because the heat loss at the slag line is substantially larger due to surface area to molten metal ratio. Therefore for smaller ladles, the benefits of improved inclusion flotation are negated by unstable slag lane. Conclusions: Argon stirring with a porous plug in 1lb steel ladles did not improve inclusion flotation, but degraded steel cleanliness by introducing large levels of TiO 2 and MnS. This was validated by quantitative inclusion measurements and Charpy impact testing. References: [1] Lalhua Wang, Hae-Geon Lee, Peter Hayes, A New Approach to Molten Steel Refining Using Fine Gas Bubbles, ISIJ International Vol 36 (1996), No 1 pp [2] Alan Cramb, The Making Shaping and Treating of Steel, Casting Volume, 11 th Edition, AISE Steel Foundation, Pittsburgh, 23. [3] Vintee Singh, Inclusion Modification in Steel Castings using Automated Inclusion Analysis, M.S. Thesis, University of Missouri Science and Technology, Rolla, 29.

5 Appendix A Comparison of Cast Keel blocks from all of the Ladle Trials Number of Inclusions (No. / mm 2 ) a. Number of inclusions per mm 2 in the cast keel bar for the porous plug Ar-stir trials. Fraction of Area Covered by Inclusions b. Fraction of area covered by inclusions in the cast keel bar for the porous plug Ar-stir trials. Amount of Oxygen and Nitrogen (ppm) b. The oxygen and nitrogen content in the cast keel bar for the porous plug Ar-stir trials. Figure 1: The inclusion content in the cast keel bars for all of the Ar-stir trails. Oxygen Nitrogen Other TiO2 CaO Al2O3 MnO MnSiO3 CaS MnS Other TiO2 CaO Al2O3 MnO MnSiO3 CaS MnS

6 Fraction of area covered by inclusions % Ca. ft/min Ladle Stir.24% Ca.28% Ca.32% Ca.43% Ca.43% Ca Ladle Stir.5% Ca.6% Ca 2 ft/min Others TiO2 CA Al2O3 MnO MnSiO3 CaS MnS Figure 2: Fraction of area covered by inclusions in the cast mold for all trials of the Ca-wire injection study. Note: The first two trials (an untreated ladle and in ladle Ar lance stir) provide a good perspective to the porous plug Ar-stir trials. Charpy Impact Energy Absorbed (ft-lbs) a. Charpy impact energy absorbed for all ladle treatments conducted in the porous plug Ar-stir trials. Charpy Impact Energy Absorbed (ft-lbs) % Ca. ft/min Ladle Stir.24% Ca.28% Ca.32% Ca.43% Ca.43% Ca Ladle Stir.5% Ca.6% Ca 2 ft/min b. Charpy impact energy absorbed for all ladle treatments conducted in the Ca-Wire injection trials. Note: The dashed lines are the maximum and minimum Charpy impact values observed in the porous plug trial and the solid line is the average Charpy impact value. Figure 3: Charpy impact energy measurements for normalized keel bars samples.