Casting and Solidification
An in situ Study of the Formation of Rare Earth Inclusions in Arsenic High Carbon Steels
2019 Volume 59 Issue 7 Pages 1259-1265
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2019 Volume 59 Issue 7 Pages 1259-1265
The application of rare earths is an effective way to stabilize residual elements in steel, such as As and P, so as to improve the performances of steel products. In situ methods were used to investigate the formation of inclusions and their stability at high temperatures in arsenic high carbon steels with additions of lanthanum. The results show that La2O3 and La2O2S started generating in molten steel and had significant difference of appearances and growth behaviors. La2O3 started with triangular particles and rapidly grew up like crystals; by contrast, La2O2S particles were always spherical or near-spherical and didn’t significantly grow up. Arsenic existed as LaAsO4 that turned out to be unstable under high temperatures. LaAsO4 decomposed and As dissolved into the matrix when the temperature was higher than 1200°C. The formation of LaAsO4 during solidification and the dissolution of As into the matrix during heat treatment can effectively avoid the local enrichment of As. Therefore, it is possible to control As distributed uniformly in steel by appropriate heat treatment process.
Residual element arsenic (As) is extremely detrimental to the quality and properties of steel products. It is easy for As, usually together with Sn, Sb, Cu, etc., to segregate to grain boundaries and phase interfaces in the hot working process, which may cause a sharp decrease of the hot workability of billets.1,2) This segregation may also lead to the temper embrittlement of steel products.3) Arsenic-induced hazards are a serious problem for steel products, especially in China due to its plenty storage of arsenic iron ores. It is impossible to fully remove As from arsenic iron ores because of the limited dearsenification ability of ironmaking process.4,5) Although both slag treatment and vacuum treatment can partly remove As from arsenic molten iron in the steelmaking process, they also result in serious pollution or massive iron loss. Currently, a mainstream solution to this problem is diluting arsenic molten iron with arsenic-free molten iron in the steelmaking process.6) It is, however, obviously not a sustainable method.
In view of the unique electronic shell structures, rare earth (RE) elements have strong affinities with oxygen and sulfur in steel and have been used to purify molten steel for decades. Since the 1970s, much fundamental work on the formation of RE inclusions has been carried out, and much relevant thermodynamic data was obtained.7,8,9,10,11,12) Some side reactions between RE elements and crucibles were also investigated.13,14) It was proved that RE-As even RE-O-S-As inclusions might form in arsenic RE steels.15,16) What kinds of inclusions may form greatly depends on the chemical compositions of steel, especially the concentrations of RE, O and S; further, the formation of arsenic inclusions depends to a large extent on RE inclusions. It was reported that arsenic existed as many kinds of inclusions in steel, such as RES·AsS,16) RE-As or RE-O-As17) and RE-As-P.18) However, the formation mechanism of RE inclusions is still controversial for the lack of thermodynamic data, especially for arsenic inclusions.
There are two main theories related to the formation of inclusions in steel. On the one hand, affinities among elements greatly determine the formation of inclusions. RE elements don’t react with As until the concentrations of O and S are low enough because of relatively low affinities between As and RE. On the other hand, nucleation modes greatly determine the generation and distribution of inclusions. The oxides, sulfides and oxysulfides of RE, generating by homogenous nucleation, are potential heterogeneous nuclei for the following arsenic inclusions. It was found that LaAsO4 existed as both La2O2S–LaAsO4 complex inclusions and single LaAsO4 particles.19) How and on what conditions will arsenic RE inclusions generate need to be clarified.
This work used in situ observation methods to directly observe the formation of RE inclusions in two arsenic high carbon steels, including the processes of homogeneous nucleation, heterogeneous nucleation and growth behavior, so as to discuss how to control the formation of these inclusions.
The raw materials are the ingots of grade 80 steel produced by an iron and steel company in southern China. This company has used some arsenic iron ores in recent years to reduce production costs. Alloys were prepared through adding La metal to the remelted ingots in a SiMo furnace. About 500 g worth of ingot was remelted in a corundum crucible at 1600°C under the protection of argon gas. After the ingot was totally remelted, La metal with a purity of 99.9 mass% was added to molten steel. An iron rod was used to stir molten steel every 10 minutes for 2 times. Molten steel was furnace-cooled to 1400°C after being heated under 1600°C for 30 minutes, then subsequently air-cooled to room temperature. The concentrations of La and O were measured with an inductively coupled plasma atomic emission spectrometer (ICP-AES) and an oxygen and nitrogen analyzer, respectively. The concentrations of the remaining elements were tested with a direct-reading spectrometer. The chemical compositions of the obtained alloys are listed in Table 1.
| No. | C | Si | Mn | P | S | T.Al | As | T.O | La |
|---|---|---|---|---|---|---|---|---|---|
| blank | 0.79 | 0.24 | 0.59 | 0.013 | 0.009 | 0.002 | 0.028 | 0.0043 | – |
| 1 | 0.82 | 0.22 | 0.60 | 0.019 | 0.001 | 0.006 | 0.041 | 0.0053 | 0.059 |
| 2 | 0.74 | 0.22 | 0.59 | 0.016 | 0.003 | 0.001 | 0.034 | 0.0046 | 0.089 |
A confocal laser scanning microscope (CLSM) was employed to in situ observe inclusions. Samples with a size of Φ7.8 mm × 3 mm were cut from the obtained alloys and polished. They were heated in a corundum crucible with an inner diameter of 8 mm and a height of 4 mm under the protection of argon gas; and then they were rapidly heated up to 1300°C and heated up to 1550°C at a heating rate of 0.5 to 1°C/s. A holding operation was carried out during the heating process when necessary. After being heated at 1550°C for 5 minutes, samples were cooled to room temperature rapidly at a cooling rate around 60°C/s. A video camera was set to record the whole process with a format of 15 frame/min. Samples were examined with a JEOL 7800F scanning electron microscope (SEM) as well as a TESCAN VEGA 3 SEM. The chemical compositions of both the surface and internal inclusions were identified with an energy dispersive spectrometer (EDS). To investigate the segregation of solutes, alcohol solution with 4 vol% HNO3 was used to erode the polished samples to observe the dendrite boundaries; and the erosion time was set to be 30 seconds.
Further in situ ageing experiments were performed to investigate the stability of inclusions at high temperatures. The method used to prepare samples was the same as mentioned above. Some typical inclusions were chosen first, near which some marks were printed with a Vickers hardness tester. To make sure that these inclusions were the correct targets, we checked their chemical compositions by SEM and EDS before ageing the samples. After being separately aged at 1350°C and 1200°C for an hour under the protection of argon gas, the same inclusions in the samples were examined with SEM and EDS again.
Figure 1 shows the formation of typical inclusions in both La-bearing steels. For 0.059 mass% La steel, some local parts of the specimen began to melt as the temperature increased. The original inclusions together with the newly generated ones floated up to the surface of molten steel, marked by type A in Fig. 1(a). These inclusions moved very fast and were attracted to be clusters (Figs. 1(b) and 1(c));20) finally, they stayed on the surface of the specimen during solidification.
Formation of typical inclusions in 0.059 mass% La steel ((a) 1436°C, (b) 1442°C, (c) 1453°C) and 0.089 mass% La steel ((d) 1388°C, (e) 1413°C, (f) 1455°C). (Online version in color.)
As La content increased to 0.089 mass%, the formation of inclusions was significantly different, as shown in Figs. 1(d) to 1(f). In the same way as the melting behavior of 0.059 mass% La specimen, some local parts began to melt and type A inclusions appeared first, marked in Fig. 1(d). However, after the alloy fully melted, some triangular inclusions, marked by type B in Fig. 1(e), formed and grew up at a high growth rate like crystals (Fig. 1(f)). By contrast, type A inclusions just aggregated together.
Figure 2 shows the morphology and EDS images of typical inclusions on the surfaces of 0.059 mass% La and 0.089 mass% La specimens, respectively. It turned out that type A inclusions (points A and C) were La–O–S ternary compounds and considered as La2O2S,19) RE2O2S was also reported the only possible RE oxysulfide in steel.7) Type B inclusions (points B and D) were La–O binary compounds and considered as La2O3, since RE2O3 was reported the most possible RE oxide.7,21) La2O2S and La2O3 showed a noteworthy difference of morphology. The former was mostly spherical or near-spherical; however, the latter started with triangles and ended up with a herringbone shape. They were also the two main inclusions formed in molten steel.
Morphology and EDS images of inclusions on the surfaces of specimens: (a) SEM image of 0.059 mass% La specimen, (b) EDS of point A, (c) EDS of point B, (d) SEM image of 0.089 mass% La specimen, (e) EDS of point C, (f) EDS of point D.
It is noted that the melted specimens became a liquid drop (see Fig. 3(a)) instead of a cylinder, owing to the surface tension of molten steel. Therefore, when inclusions floated up to the surface of the liquid drop, they gathered in a very small area at the top surface. It was very hard to capture the images of inclusions on the surface of the totally remelted specimen with 0.059 mass% La, because just a limited number of inclusions formed and moved rapidly. That was why we didn’t capture any images of type B inclusions during in situ observation. By contrast, a large number of inclusions generated in 0.089 mass% La specimen that made them easier to be observed. Besides, gathering in a small area contributed to the rapid growth of La2O3.
Morphology and EDS mapping images of internal inclusions in 0.089 mass% La specimen: (a) a profile of the specimen, (b) a magnified map of the specimen. (Online version in color.)
Except for the inclusions generated in molten steel, some other inclusions formed during solidification and cooling process. Figure 3 shows the morphology and EDS mapping images of internal inclusions in 0.089 mass% La specimen. Unlike inclusions on the surface, these internal inclusions were arsenic RE ones, consisting of La2O2S cores and La–O–As covering. Besides, some single La–O–As ternary compounds were also discovered and identified as LaAsO4, of which the detail was reported in the previous work.19) It can be inferred that LaAsO4 precipitated first on the surface of La2O2S, because heterogeneous nucleation was more favored over homogeneous nucleation. As the temperature went down, single LaAsO4 particles also precipitated by homogeneous nucleation.
3.2. Effect of Solute Segregation on Formation of Arsenic RE InclusionsIt is reasonable to consider that the formation of LaAsO4 originates from the area of solute segregation during solidification. To find the original solidified dendrite boundaries more conveniently, the solidification microstructures near the surface of the samples were chosen. This is because the solidification of droplet-like samples starts easily on the basis of crucible bottom that makes the solutes segregate to the upper liquid steel; thus the solute segregation can be observed more easily at the upper part.
Figure 4 shows the solidification microstructure morphologies and solute distributions at dendrite boundaries of La-free and 0.089 mass% La samples. Many dendrite tips were found on the surface of both samples (Figs. 4(a) and 4(e)). The boundaries of these dendrites are supposed to be the most possible positions where solute segregation takes place. As expected, S and As were found to be segregated at dendrite boundaries in La-free sample (Figs. 4(b) to 4(d)). However, no dramatic segregation of S and As was discovered in 0.089 mass% La sample. Instead, some La2O2S particles, near dendrite boundaries, were discovered and As-rich layers, considered as LaAsO4, were found at their outer layers (Figs. 4(f) to 4(h)).
Solidification microstructure morphologies and element distributions at dendrite boundaries: (a) and (b) the profile of La-free sample and its local magnified map, (c) and (d) the line scanning positions and maps of L1, (e) and (f) the profile of 0.089 mass% La sample and its local magnified map, (g) and (h) the line scanning maps of L2 and L3. (Online version in color.)
The oxides, sulfides and oxysulfides of La are supposed to be the first La-bearing inclusions that form in La-bearing steels, and which forms first depends on the initial chemical compositions.19) These inclusions are, of cause, the potential heterogeneous nucleation cores of As-bearing inclusions. The concentration of As is considered the restrictive factor for the formation of LaAsO4 in steel. The segregation of As dramatically increased its concentration at dendrite boundaries, where became the most possible positions for LaAsO4 to initially generate. Also, the segregation of S increased its concentration at dendrite boundaries, making more La2O2S particles distributed along these boundaries, except for the ones floated up to the surface of specimen. Owing to the heterogeneous nucleation effect, LaAsO4 preferred to precipitate on the surface of La2O2S. Thus the complex La2O2S–LaAsO4 inclusions generated. The size of single LaAsO4 inclusions around 1.5 to 4 μm also indicated that LaAsO4 generated more likely during solidification, because it is generally very hard to form such big inclusions in solid steel owing to poor dynamic conditions. Therefore, the formation of LaAsO4 started initially at dendrite boundaries, and of cause, modified the existing state of the segregated As in steel.
The effect of La concentration on the homogeneity of As, from the point of view of the formation of arsenic RE inclusions, was discussed. In the previous work, we investigated inclusions in five heats of steel with the highest La concentration of 0.089 mass%.19) No arsenic RE inclusions formed in 0.043 mass% La steel but plenty of La2O2S–LaAsO4 inclusions formed as La concentration increased to 0.059 mass%. Therefore, it is referred that a value between 0.043 mass% and 0.059 mass% is the critical value for La to act the role on inhibiting the segregation of As. This critical value, of cause, is related with the initial concentrations of O, S and As in steel. Further investigation is needed to determine the quantitative relationship between La concentration and the homogeneity of As.
3.3. Hot Stability of Arsenic RE InclusionsTo further clarify the formation of LaAsO4, samples containing La2O2S–LaAsO4 complex inclusions were heated for an hour at different temperatures. The covering LaAsO4 disappeared after being heated at 1350°C for an hour, as shown in Fig. 5. Obviously, LaAsO4 decomposed and La, As and O dissolved into the matrix of steel. Further, for the specimen after being heated at 1200°C for an hour, many cluster inclusions generated around La2O2S cores except for the disappearing of LaAsO4, as shown in Fig. 6. The decomposition of LaAsO4 suggested that LaAsO4 was unstable under the temperature higher than 1200°C.
Morphology and EDS mapping images of inclusions in 0.089 mass% La steel: (a) before heated, (b) after heated at 1350°C, (c) and (d) magnified maps of the target inclusion before and after heated. (Online version in color.)
Morphology and EDS mapping images of inclusions in 0.089 mass% La steel: (a) before heated, (b) after heated at 1200°C, (c) and (d) magnified maps of the target inclusion before and after heated. (Online version in color.)
Further EDS line scanning results show that the newly generated inclusions under 1200°C were mostly SiO2 and slightly Al2O3, as shown in Fig. 7. Although both Si and Al have lower affinities with O than that of La, they can still react with O and precipitate at the existed La2O2S particles as long as the local concentration of La was low enough. During normal cooling process, La would continuously react with O, S and As, and then La2O2S or LaAsO4 generated. During the ageing period under 1200°C, however, the existed LaAsO4 would dissolve into the matrix. After that the following formation of inclusions was primarily associated with La, O, S, Si and Al. In solid steel, the diffusion rates of interstitial atoms are far larger than that of solid solution atoms. Oxygen is an interstitial atom but lanthanum a solid solution atom. This is to say, with the continuous formation of La2O2S, there would be a sharp decrease of La concentration in the local area around La2O2S particles because the distant La atoms were difficult to reach. By contrast, O atoms had a fast diffusion speed and were easier to reach the La-depleted region, contributing to the formation of SiO2 and Al2O3 on the surface of La2O2S particles. For inner inclusions (Fig. 7(c)), the growth of SiO2 and Al2O3 filled the positions that arsenic atoms had occupied. For inclusions on the surface (Fig. 7(a)), the growth of SiO2 and Al2O3 was unrestricted and they didn’t need to fill the positions that had belonged to arsenic atoms; thus the morphologies of inclusions have become crown-like and some gaps generated.
Surface and profile of inclusions in 0.089 mass% La sample heated under 1200°C: (a) the SEM image of the surface and (b) its line scanning maps, (c) SEM image of the profile and (d) its line scanning maps. (Online version in color.)
The formation sequences of the oxides, oxysulfides and sulfides of La were mostly determined by the concentrations of La, O and S in steel. It was reported that the inclusions were mainly RE2O2S when S concentration was in the range of 10 to 100 times that of O.9) In this research, S concentrations in both La-bearing steels were far less than 10 times that of O, but the first inclusions observed turned out to be La2O2S (see Figs. 1 and 2), which was controversial to Lu et al.’ work.9) The densities of La2O3 and La2O2S, which are 6.5 and 5.8 kg/cm3 respectively, are an important factor accounting for this contradiction. The lower density of La2O2S contributed to a faster floatation speed, thereby La2O2S particles were easier to float up to the surface of molten steel. But that didn’t mean the formation of La2O2S was ahead of La2O3.
After the specimen totally melted, the growth behaviors of La2O3 and La2O2S strongly suggested that the former were actually easier to generate. As shown in Fig. 8, the formation of inclusions in 0.089 mass% La steel was chosen to describe this process since there were more inclusions to be easily observed. We assumed that Fig. 8(a) showed the starting point when the heating temperature reached 1500°C. There were already some La2O3 crystals floating on the surface of molten steel, typically marked at positions 1 and 2 with red arrows. In the following 4 seconds, La2O3 crystals continuously grew up at the interface between the original La2O3 crystals and molten steel, typically marked at position 3 (Fig. 8(b)). After another 12 seconds, some La2O2S particles appeared at the interface (Fig. 8(c)). After that the amount of La2O2S particles continuously increased (Fig. 8(d)). Finally, both La2O3 crystals and La2O2S particles stayed on the surface of specimen after solidification, and no other inclusions were discovered on the top surface. It can be concluded that La2O3 generated ahead of La2O2S overall.
Growth process of inclusions on the surface of molten steel with 0.089 mass% La at 1500°C. (Online version in color.)
The formation of RE inclusions in arsenic high carbon steels was successfully investigated by in situ methods. It turned out that both the nucleation and growth behavior of La2O3 and La2O2S were significant different. La2O3 started with triangular particles, and dramatically grew up like crystals when collided together; by contrast, La2O2S always showed a spherical or near-spherical shape and didn’t significantly grow up.
In situ observation proved that arsenic inclusions LaAsO4 were unstable under high temperatures. They decomposed and As dissolved into the matrix when the temperature was higher than 1200°C. Therefore, the mechanism of La on the existing state of As in steel can be concluded as follows: (i) the formation of LaAsO4 during solidification can effectively avoid the formation of low-melting-point As-rich phase; (ii) the decomposition of LaAsO4 and the following dissolution of As into the matrix during heat treatment can effectively make As uniformly distributed in steel.
This research is financially supported by National Natural Science Foundation of China (No. 51704051), Foundation of Key Laboratory for Ecological Metallurgy of Multimetallic Mineral (Ministry of Education) (No. NEMM2018001) and Chongqing Postdoctoral Funds (No. Xm2017192).
