Effect of Cooling Rate on the Evolution of Nonmetallic Inclusions in a Pipeline Steel
ZHANG Yuexin1, WANG Jujin2, YANG Wen1(), ZHANG Lifeng2()
1School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China 2School of Mechanical and Materials Engineering, North China University of Technology, Beijing 100144, China
Cite this article:
ZHANG Yuexin, WANG Jujin, YANG Wen, ZHANG Lifeng. Effect of Cooling Rate on the Evolution of Nonmetallic Inclusions in a Pipeline Steel. Acta Metall Sin, 2023, 59(12): 1603-1612.
Controlling nonmetallic inclusions in steels is critical during the steelmaking process. Temperature affects the chemical equilibrium between steel and the inclusions, the composition of the inclusions changes with changes in temperature. During the solidification and cooling processes, the cooling rate is a significant factor affecting the temperature. Therefore, the composition of nonmetallic inclusions transforms during the solidification and cooling of steels. To study the evolution of the inclusion composition in pipeline steel at cooling rates of 800, 600, 400, 200, 100, and 5oC/min, high-temperature confocal scanning laser microscopy was employed to accurately control the temperature during the cooling process. The thermochemical software FactSage was employed to reveal the theoretical basis of the transformation of the inclusion composition. A kinetic model for the evolution of the inclusion composition in pipeline steel during the cooling process was established, and the effect of inclusion diameter and cooling rate on the transformation was analyzed. The results revealed that with the decrease in the cooling rate, the Al2O3 content in the inclusions increased from 66.33% to 75.06%, the CaS content increased from 1.07% to 10.55%, and the CaO content decreased from 28.27% to 11.24%. Further, the MgO content decreased from 4.33% to 3.15% during the cooling process. The number densities of the inclusions were 76.15 and 15.28 mm-2 at cooling rates of 800 and 5oC/min, respectively. As the cooling rate decreased, the average diameter of the inclusions first decreased from 2.09 to 1.62 μm and subsequently increased. The thermodynamic equilibrium composition of the inclusions in the molten steel was 41.71%CaO-50.76%Al2O3-6.50%MgO-1.03%SiO2. With a decrease in temperature, inclusions transformed from Al2O3-CaO-MgO to CaS-Al2O3-MgO-(CaO). The cooling rate had little effect on the MgO and Al2O3 contents in the inclusions. The inclusion diameter and cooling rate had an apparent influence on the CaO and CaS contents in the inclusions. The critical cooling rate at which the CaS content became greater than the CaO content was impacted by the inclusions' diameter. The critical cooling rates for inclusions with diameters of 1 and 2 μm were approximately 400 and 100oC/min, respectively, whereas the rates were much smaller than 1oC/min for inclusions with diameters larger than 5 μm.
Fig.1 Schematic of the temperature rise and drop processes of samples
Fig.2 Compositions of inclusions at CaO-Al2O3-CaS (a1-g1) and CaO-Al2O3-MgO (a2-g2) ternary composition diagrams in the blank sample (a1, a2), and pipeline steels at cooling rates of 800oC/min (b1, b2), 600oC/min (c1, c2), 400oC/min (d1, d2), 200oC/min (e1, e2), 100oC/min (f1, f2), and 5oC/min (g1, g2) (Dmax—maximum diameter of inclusions, Dave—average diameter of inclusions. When CaS% > MgO%, inclusions were projected into CaO-Al2O3-CaS ternary composition diagram, otherwise inclusions were projected into CaO-Al2O3-MgO ternary composition diagram. Each circle in the figure represents an inclusion, and the size of the circle represents the diameter of the inclusion)
Fig.3 Variations of average composition of inclusions at different cooling rates
Fig.4 Variations of the number density and average diameter of inclusions at different cooling rates
Fig.5 Distributions of inclusion number density with diameter at cooling rates of 800oC/min (a), 600oC/min (b), 400oC/min (c), 200oC/min (d), 100oC/min (e), and 5oC/min (f)
Fig.6 Evolutions of phase (a) and thermodynamic composition of inclusions (b) in the pipeline steel over temperature during steel solidification and cooling (w(i)—mass fraction of element i, T.—total, l—liquid, s—solid)
Element
In liquid
In δ
In γ
Al
3.5 × 10-09
5.9 × exp(-241186 / (RT )) / 10000
5.15 × exp(-245800 / (RT )) / 10000
Mg
3.5 × 10-09
0.76 × exp(-224430 / (RT )) / 10000
0.055 × exp(-249366 / (RT )) / 10000
Ca
3.5 × 10-09
0.76 × exp(-224430 / (RT )) / 10000
0.055 × exp(-249366 / (RT )) / 10000
S
4.1 × 10-09
4.56 × exp(-214639 / (RT )) / 10000
2.4 × exp(-223426 / (RT )) / 10000
O
2.7 × 10-09
0.0371 × exp(-96349 / (RT )) / 10000
5.75 × exp(-168454 / (RT )) / 10000
Table 1 Diffusion coefficients of dissolved Al, Mg, Ca, S, and O in liquid, δ, and γ steel[26-28]
Fig.7 Comparisons of calculated and measured values of inclusion composition with 2 μm diameter at different cooling rates
Fig.8 Variations of the composition of inclusions with the cooling rates and their diameters (a) CaO (b) CaS (c) Al2O3 (d) MgO
Fig.9 Variations of the composition of inclusions with diameters of 1 μm (a), 2 μm (b), 5 μm (c), and 7 μm (d) with cooling rates
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