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Vol.11 No.1 January 2014Research & DevelopmentCHINA FOUNDRY
Formation mechanism of shrinkage and large inclusions of a 70 t 12Cr2Mo1 heavy steel ingotLiu Hongwei, *Fu Paixian, Kang Xiuhong, and Ma XiaopingShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
The internal quality of heavy ingots will influence the quality of the final forging products. In
practical production, some forgings are frequently rejected because of serious defects of the ingots [1-2]. With the development of large equipment used under high temperature and pressure, in order to ensure the reliability of the forging production in operation, the requirement for fine steel ingots is increased. For a steel ingot, the main internal defects include central shrinkage cavity and porosity, macrosegregation and large inclusions at the bottom of the ingot [3-4]. Generally, shrinkage defects occur in the central part of the ingots and seriously influence the quality of the products if they can not be eliminated by forging [5]. As a means, mold optimization through computer simulation was applied to eliminate the shrinkage defects, but the shrinkage defect criterion for a certain material was extremely crucial for obtaining accurate simulation result, which can only be obtained by combining sectioning investigation with solidification process simulation [6]. Macrosegregation
Abstract: Shrinkage cavities and large inclusions are serious internal defects of heavy steel ingot and influence the quality of subsequent forgings. In order to remove these two types of defects, a 70 t 12Cr2Mo1 heavy ingot fabricated by vacuum carbon de-oxidation process was sectioned and investigated by means of structure observation and EDS analysis. To further study the forming mechanism of shrinkage and inclusion defects and find possible solutions, simulation on pouring and solidification processes was also carried out using Fluent and ProCAST software, respectively. Results show that the shrinkage defects do not appear in the middle-upper part of the ingot. The critical value of shrinkage cavity criterion is ascertained as 0.013 on the basis of sectioning investigation and simulation results, which can be used in computer simulation to predict and avoid shrinkage defects in production of 12Cr2Mo1 ingots with different weights. However, large inclusions are found at the bottom of the ingot body. The bad thermal conditions of the ingot surface and large amount of entrained slag are the main origin of the large inclusions. The simulation result of the pouring process shows that large inclusions may be eliminated by combined measures of improving the top thermal condition and controlling the height of rudimental molten steel in the ladle to above 300 mm.
Key words: 12Cr2Mo1 heavy ingot; segregation; shrinkage defect; large inclusions
CLC numbers: TG142 Document code: A Article ID: 1672-6421(2014)01-046-06
in a mold ingot mainly includes positive segregation, negative segregation, A-segregation and V-segregation [7]. To reduce macrosegregation, advanced vacuum carbon deoxidation (VCD) refining technology was applied during steelmaking but only in some special steels due to operation difficulty [8]. More accumulated inclusions were observed in A-segregation or V-segregation than in any other place. Especially, there are many MnS inclusions. These inclusions may become the origins of cracks and endanger the safe operation of the product. Accumulated large inclusions in the zone of the deposit cone are serious metallurgical problem in the bottom-pouring ingot, which results from the “falling of crystal rain” from the top at the early stage of solidification [9]. The same defect was also found in top-pouring ingots, and few studies on this aspect were found in open literatures. Owing to its close relationship with the final product quality, it is necessary to clarify the formation mechanism of large inclusions at the bottom of top-pouring ingots.
In this study, a detailed sectioning investigation was performed on a 70 t 12Cr2Mo1 ingot manufactured by VCD refining technology to study the central shrinkage defects and inclusions of the ingot. To further study the forming mechanism and conditions of shrinkage and inclusion defect and find a practical possible solution
*Fu PaixianMale, born in 1979, Ph.D, Associate Professor. His research interests mainly focuse on the control of the internal qualities of steel ingots and castings. E-mail: [email protected]
Received: 2013-01-15 Accepted: 2013-08-11
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Vol.11 No.1 January 2014Research & Development CHINA FOUNDRY
to the problem, the simulation on pouring process by Fluent software and solidification process by ProCAST software was carried out.
1 Experimental procedure The measured chemical compositions of the 12Cr2Mo1 steel are listed in Table 1. The melting and pouring process was EAF (Electric Arc Furnace) → LF (Ladle Furnace) → VCD (Vacuum Carbon Deoxidation) → MSD (Mold Stream Degassing), and top-pouring process was used. The pouring temperature was 1,570 ℃. Figure 1 shows a sketch of the ingot with the main parameters of H/D = 1.65 (H = 2,850 mm, height of ingot; D = 1,727.5 mm, mean diameter of ingot) and TP = 7% (taper of ingot body). The selection of these parameters were mainly based on the consideration of gaining a sound ingot body and a high utilization rate of ingot according to practical experiences. It is known that the higher the H/D value, the higher the utilization rate of ingot, but the more the shrinkage porosities and cavities at the same time.
Table 1: Chemical compositions of 12Cr2Mo1 steel (wt.%)
Fig. 1: Sizes of ingot (mm) and positions of samples
Fig. 2: Test positions for chemical composition and inclusions in sample B
1000
410
Hot top
Ingot body
Tail cone of ingot
A
B
200
H
C Si Mn Ni Cr Mo P S
0.147 0.051 0.33 0.090 2.240 0.890 0.010 0.005
Two large samples, as shown in Fig. 1, were cut separately using high temperature flame from the ingot. Sample A was located at middle-upper part of the ingot about 200 mm from the hot top, where shrinkage cavities and porosities often occur. Sample B was located at the bottom of ingot, where large inclusions often occur. The longitudinal sections of samples from the center of the ingot were prepared for investigation. To observe the segregation and shrinkage defects of the ingot, macro examinations (sulfur print and cold acid etching) were performed; the SEM morphology observation and EDS analysis of inclusions were carried out. On the eleven sampling positions of sample B, as shown in Fig. 2, the chemical composition was measured, and detailed inclusions analysis was conducted.
2 Simulation procedureThe solidification process of the ingot was simulated using the commercial casting software ProCAST with Porosity Fig. 3: Macro-structure of sample A with cold acid etching
Model, which can predict the shrinkage defects. In simulation, the initial conditions and thermal boundary condition were as follows: the pouring temperature was 1,570 ℃, the liquidus temperature was 1,518 ℃, and the solidus temperature was 1,470 ℃. Heat transfer coefficients between the iron mold and the outside and the iron mold and steel were 10 W∙m-2∙K-1 and 1,500 W∙m-2∙K-1, respectively. The heat transfer coefficients between the insulation board and insulation brick and the board and iron mold are both 20 W∙m-2∙K-1 [11].
The pouring process of molten steel was simulated using VOF (volume of fluid) model of Fluent software, which can simulate two or more immiscible fluids by solving a single set of momentum equations and tracking the volume fraction of fluids. Based on practical conditions, the height of molten steel and slag was set as 2,295 mm and 150 mm respectively, and other space above the slag, in the ladle, was set as air. The initial conditions were that every phase keeps stationary state and there was no heat transfer. In the simulation, the residual of the solution was defined as below 10-6, which assures a good numerical convergence.
3 Results and analysis3.1 Sectioning investigation on middle-upper
part of ingot bodyFigure 3 shows the macro-structure of sample A located at middle-upper part of the ingot body after cold acid etching. In the macro-structure, neither A-segregation nor V-segregation is observed. Besides, no shrinkage cavity or porosity is observed. A few uniformly distributed inclusions are observed in sample A. The SEM and EDS analysis results in Fig. 4 show that the inclusions are mainly complex oxysulfides composed of Al2O3 and MnS, and the size of the inclusions is commonly below 5 µm.
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Vol.11 No.1 January 2014Research & DevelopmentCHINA FOUNDRY
Fig. 6: Large conglomerated inclusions at bottom of ingot (sample B)
Fig. 7: Partial morphology of bottom large inclusion in position 2 of sample B [(a) and (b) are photos observed in different selected typical parts]
Fig. 5: Super-large inclusion in sample B with cold acid etching (a) and sulphur print (b)
Fig. 4: SEM morphology (a) and EDS analysis (b) of inclusion in sample A
(a) (b)
(a) (b)
(a) (b)
3.2 Sectioning investigation on bottom part of ingot body
For sample B located at bottom part of the ingot body, the macro-structure in Fig. 5 shows that there is a super-large inclusion in the sample; it is located at position 2 in Fig. 2, i.e., in central zone of the bottom of ingot body. The size of the inclusion is about Ф20 mm. In the position 10 mm under the super-large inclusion, there is an area of approximately 45 mm × 55 mm, which includes many accumulated large inclusions (see Fig. 6).
Because the size of the above-mentioned inclusion was so big, its typical part was selected for the analysis of
compositions in position 2, as shown in Fig. 7. It was observed that the inclusion is composed of white, gray, and black different zones. The compositions of each zone are listed in Tables 2 and 3. From Table 2, it can be seen that the white zone is Fe particle, the gray zone is spinel mainly consisted of magnesium, chromium and aluminum oxides, and the black zone includes Si, Ca, Na, Mn in addition to the chemical composition of the gray zone. From Table 3, the compositions of the white and gray zones in Fig. 7(b) are similar to the results in Fig. 7(a), but the black zone is mainly composed of MgO. The inclusions in other positions were also tested. There mainly are two types of inclusions in these samples.
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Table 2: Compositions of partial large inclusions in Fig. 7(a) (wt.%)
Table 3: Compositions of partial large inclusions in Fig. 7(b) (wt.%)
Elements O Na Mg Al Si Ca Cr Mn Fe Total White zone - - - - - - - - 100 100
Gray zone 49.85 - 9.10 16.13 - - 23.44 - 1.48 100
Black zone 47.75 5.74 3.04 7.68 18.02 12.28 0.50 4.71 0.28 100
Elements O Mg Al Ti Cr Mn Fe Total White zone - - - - - - 100 100
Gray zone 49.20 15.61 27.12 0.70 7.36 - - 100
Black zone 45.26 53.48 - - 0.36 0.90 - 100
Fig. 8: Morphology of global inclusion of above 50 μm in size (a) and its composition in dark zone (b) and gray zone (c)
(b)(a) (c)
Fig. 9: Irregular morphology of inclusions below 10 μm in size (a) and its composition in dark zone (b) and gray zone (c)
(b)(a) (c)
One type is global inclusions with above 50 μm in size, as shown in Fig. 8(a); their chemical compositions are CaO-SiO2-Al2O3 in the dark zone and Al2O3-MgO-Cr2O3 in the gray zone, as shown in Figs. 8(b) and (c), and they mainly distribute in positions 1, 3, and 6 in Fig. 2. The other type is irregular morphology inclusions below 10 μm in size, as shown in Fig. 9; their chemical compositions are mainly MnS in the dark zone and Al2O3-MgO-Cr2O3 in the gray zone, as shown in Figs. 9(b) and (c), and they are mainly distributed in other positions. The
amount of the complex oxide inclusions in Fig. 8 is more than that of the oxysulfide inclusions in Fig. 9. The results above indicate clearly the distribution, chemical composition, size of inclusions at the bottom of the ingot, and also show that there are large numbers of inclusions above 50 μm accumulated in the central zone at the bottom of the ingot. Owing to these large inclusions, the zone cannot be used for forgings, which decreases the material's utilization ratio of the ingot.
3.3 Analysis on shrinkage defects of experimental ingot
Formation of shrinkage defects is closely related with solidification process of the ingots. Based on the chosen mold design, solidification process of the experimental ingot was visualized by ProCAST software. The change of solidification front at different solidification times is shown in Fig. 10. In
the early stage of the solidification, the solidification front shows a U shape, which is favorable to the feeding of melt, as shown in Fig. 10(a). With the development of solidification, the solidification front gradually develops as a V shape. The V shape reduces the feeding capability of molten steel. The joint angle of the V shape in the solidification front is smaller, between 1/3 position of ingot body [see Fig. 10(b)]
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and 4/5 position [see Fig.10(c)], than in other zones. Thus the shrinkage defects easily occur in this zone with bad feeding capability. But in this zone, the simulation results show that the mush area of the solidification front is not large, so serious shrinkage cavities do not occur. After 4/5 position, the joint angle of the V shape increases, which means that there is enough thermal energy transferred from the hot top to ensure feeding capability at the late stage of solidification. Moreover, the result shows shrinkage defects, predicted by the porosity model in ProCAST, would not occur when the value of criterion of shrinkage defects was above 0.013, as shown in Fig. 11. Since the value is very small, it also indicates the defects would not be serious. In a word, above-analyzed results imply the chosen mold design is suitable and could ensure a good central quality of the experimental ingot. This is agree well with the sectioning investigation result of sample A that there is no shrinkage defects in middle-upper part of the ingot.
Based on the sectioning and simulation results, the critical value of criterion of shrinkage defects was determined as 0.013. Through application of the criterion in computer simulation, shrinkage defects of 12Cr2Mo1 ingots with different tonnage weights can be predicted and avoided by suitable mold design, which is very significant for the future simulation and production of sound 12Cr2Mo1 ingots.
3.4 Formation mechanism of large inclusionsBased on the above results for sample B, the inclusion above 50 μm is mainly composed of CaO, SiO2, Al2O3 and MgO. These oxides are the main composition of slag and flux from the top of the ingot. Besides, Na2O is found in the super-large inclusions in Fig. 7, which demonstrates that flux is one of the origins of the large inclusion [11]. Moreover, the high content of MgO should come from the liner bricks suffering serious high erosion in the ladle. The detached MgO floats to the surface of the molten steel, and combines with other slag. It is crucial to understand why the slag and flux appear in the bottom of the ingot. Thermal condition of the top molten steel is considered to have great influence. The ingot was poured under the condition of vacuum. After the melt was poured into the mold, the condition of vacuum was cancelled, and the flux was added. Owing to the heat loss on the surface of the ingot, the molten steel froze quickly. When the flux burned on the surface, the thermal energy will be enough to re-melt the frozen shell. At this moment, a large amount of “crystal rain” falls from the remelted surface shell. Suzuki [12] has demonstrated the fact by experiment that solidified crystals fall to the bottom of the ingot from the top. If a great deal of slag is frozen into the shell, the burned product of flux may react with the slag; and “crystal rain” may take them together to the bottom of the ingot. Through the above analysis, the bad thermal condition of the ingot surface and large amounts of entrained slag are the main reasons for large inclusions at the bottom of the ingot. In order to avoid the flux entrapped into the ingot, excepting for improving the thermal condition on the surface by suitable selection of pouring temperature and flux, the slag must be controlled not to be trapped into the ingot.
In order to control the entrapment of the slag, the pouring process of molten steel in ladle was reappeared by Fluent software. Figure 12(a) shows the initial distribution of molten steel. When the sliding nozzle is opened, the molten steel flows out with high velocity from the outlet at the bottom of the ladle, as shown in Fig. 12(b). When molten steel is below 300 mm in ladle, the slag above the sliding nozzle will be entrapped into the molten steel [see Fig. 12(c)]. Because of the strong down-flow of molten steel, the entrapped slag flows together with the molten steel from the ladle.
Based on the simulation results obtained using Fluent, through controlling the residual molten steel to have a height above 300 mm in the ladle can avoid the slag being entrapped into the mold. The combined measures of improving the top thermal condition and controlling the height of rudimental molten steel may be effective in solving the problem of large inclusions at the bottom of the ingot in practical production.
4 ConclusionsThe internal quality of the 70 t 12Cr2Mo1 steel ingot produced by vacuum carbon de-oxidation (VCD) process was investigated by detailed sectioning investigation including analysis on central shrinkage cavity and large inclusions at the bottom of the ingot.
Fig. 10: Evolution of solidification front: 3.6 h (a), 5.8 h (b), 7.7 h (c)
Fig. 11: Simulated central shrinkage defects
(a) (b) (c)
1518.00
1514.80
1511.60
1508.40
1505.20
1502.00
1498.80
1495.60
1492.40
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1486.00
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1479.60
1476.40
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0.013200
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Cri.
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Vol.11 No.1 January 2014Research & Development CHINA FOUNDRY
Fig. 12: Simulation results of pouring process: initial distribution of molten steel (red), slag (green), and air (blue) (a); starting velocity (b); slag entrapment (c)
Moreover, by numerical simulation, the forming mechanism of shrinkage and slag entrapment in the ladle were further studied. Some important conclusions are summarized as follows:
(1) The 12Cr2Mo1 large steel ingot can be manufactured through the EAF → LF → VCD → MSD process. Under the experimental conditions, no shrinkage cavity or porosity forms in the middle-upper part of the ingot.
(2) In this experiment, the critical value of criterion of shrinkage cavity occurrence is ascertained as 0.013 by means of the shrinkage porosity model in ProCAST combined with sectioning investigation results of sample in middle-upper part of the ingot, which can be used for future simulation and production of sound 12Cr2Mo1 ingots.
(3) The large inclusions are mainly composed of slag and flux, which are carried together with “remelted surface shell” from the hot top of the ingot in the early stage of solidification. The bad thermal conditions of the ingot surface and large amounts of entrained slag are the main reasons for large inclusions at the bottom of the ingot. The simulated results show that the large inclustions may be eliminated by combined measures of improving the top thermal condition and controlling the height of rudimental molten steel in ladle to above 300 mm.
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7.00e+03
6.65e+03
6.30e+03
5.95e+03
5.60e+03
5.25e+03
4.90e+03
4.55e+03
4.20e+03
3.85e+03
3.50e+03
3.15e+03
2.80e+03
2.45e+03
2.10e+03
1.75e+03
1.40e+03
1.05e+03
7.01e+02
3.51e+02
1.22e+00
7.11e+00
6.75e+00
6.40e+00
6.04e+00
5.68e+00
5.33e+00
4.97e+00
4.62e+00
4.26e+00
3.91e+00
3.55e+00
3.20e+00
2.84e+00
2.49e+00
2.13e+00
1.78e+00
1.42e+00
1.07e+00
7.11e-01
3.55e-01
0.00e+00
7.00e+03
6.65e+03
6.30e+03
5.95e+03
5.60e+03
5.25e+03
4.90e+03
4.55e+03
4.20e+03
3.85e+03
3.50e+03
3.15e+03
2.80e+03
2.45e+03
2.10e+03
1.75e+03
1.40e+03
1.05e+03
7.01e+02
3.51e+02
1.22e+00
(a) Contours of density (kg·m3) (b) Contours of velocity magnitude (m·s-1) (c) Contours of density (kg·m3)
This research is financially supported by the Program of National Technological Cooperation and Communication (Project 2010 DFR 70640), and Chinese National S&T Major Project (2011ZX06004-016).
