2015 Volume 55 Issue 5 Pages 1044-1047
Since the Cu content in steel causes hot shortness, it is important to understand the behavior of Cu during high-temperature oxidation in order to control the precipitated Cu. This study examined Cu distribution during the oxidation of steel. The oxidation tests revealed that precipitated Cu existing in the scale/steel interface was absorbed into the Fe3O4 layer or evaporated into the atmosphere as Cu. Then, a method proposed to suppress hot shortness was tested by oxidation-tensile tests at high temperature and the method was proven to be effective.
Scrap recycling reduces the burden on the environment. However, the recycling of steel scraps accumulates Cu within such steel resources, which is difficult to be removed during the steel making processes and can cause the problem of hot shortness during hot-rolling processes. When steel slabs are reheated in furnaces, Cu is enriched at the scale/steel interface and precipitated as liquid metal. Following the hot-rolling process, liquid Cu penetrates into the steel grain boundaries and causes cracks at the steel surface. However, the addition of certain elements, such as nickel, silicon, boron, carbon, and phosphorus, is reported to be effective in controlling this problem.1,2,3,4,5,6) Furthermore, controlling the reheating conditions, such as temperature and the atmospheric water vapor concentration in the atmosphere, also contribute to resolving this problem.7,8) Therefore, it is important to understand how Cu is distributed when steel is oxidized. The author has previously reported that Cu is distributed in the Fe3O4 layer and precipitates along FeO grain boundaries during scale formation.9) Hot shortness is suppressed if more Cu is removed from the scale/steel interface, while the scale structure formed at high temperature is influenced by atmospheric conditions. During this study, a method of suppressing the surface hot shortness utilizing the Cu behavior during oxidation was applied.
A section of steel containing about 0.1 mass % Cu was used for the oxidation tests, with the analyzed compositions of the material shown in Table 1. Specimens were cut to rectangular shapes of 30 mm × 30 mm × 4 mm, and oxidation was performed in an infrared heating furnace. The oxidation test conditions are shown in Table 2. Here, the variable experimental parameter is the O2 concentration in the atmosphere. Rod samples with a 10 mm diameter were used for the oxidation-tensile tests, and the analyzed compositions of the material are shown in Table 1. A tensile test machine with an infrared furnace was used for the oxidation-tensile tests, in which tensile tests were carried out at a high temperature continuously after oxidation. The tensile tests were performed at 1150°C with a tension speed of 10 mm min−1. The oxidation-tensile test conditions are shown in Table 3.
| C | Si | Mn | P | S | Ni | Cu | Al | Ti | N | |
|---|---|---|---|---|---|---|---|---|---|---|
| Oxidation test | 0.0020 | 0.009 | 0.067 | 0.012 | 0.0065 | 0.038 | 0.096 | 0.039 | 0.037 | 0.0020 |
| Oxidation-tensile test | 0.0033 | 0.008 | 0.069 | 0.012 | 0.0079 | 0.020 | 0.28 | 0.033 | 0.035 | 0.0011 |
| No. | Conditions | Experimental procedure |
|---|---|---|
| A | Base condition | (1200°C, 2%O2+12%H2O+bal.N2) ×120 min. oxidation |
| B | Low O2 condition | (1200°C, 12%H2O+bal.N2) ×120 min. oxidation |
| C | High O2 condition | (1200°C, 80%O2+12%H2O+bal.N2) ×120 min. oxidation |
| No. | Conditions | Experimental procedure |
|---|---|---|
| D | Low O2 condition | (1200°C, 12%H2O+bal.N2) ×60 min. oxidation → Tensile test (1150°C) |
| E | Base condition | (1200°C, 2%O2+12%H2O+bal.N2) ×60 min. oxidation → Tensile test (1150°C) |
| F | High O2 condition | (1200°C, 5%O2+12%H2O+bal.N2) ×60 min. oxidation → Tensile test (1150°C) |
| G | High O2 condition | (1200°C, 80%O2+12%H2O+bal.N2) ×60 min. oxidation → Tensile test (1150°C) |
The Electron Probe Micro-Analysis (EPMA) results under Condition A, in which the oxidation is performed in atmosphere containing 2% O2, are shown in Fig. 1. Cu exists not only in the scale/steel interface, but also in the upper part of the Fe3O4 layer. Cu is enriched by oxidation of steel, because Fe is consumed at the surface of the steel by the oxidation reaction and nobler metal of Cu is not oxidized and remains inside the steel. The detailed EPMA line analysis near the scale/steel interface is shown in Fig. 2(a). The enriched Cu amount at the interface is 0.075 mg cm−2. In order to calculate the value firstly enriched Cu concentrations are integrated at the Cu enriched area, where Cu concentration is higher than that at the inner region of the sample, from the EPMA analysis. The obtained value indicates the area of the gray region shown in Fig. 2(a). The enriched Cu amount is obtained by multiplying the value by the density of steel.
The cross section of the oxide scale oxidized under the base oxidation condition (Condition A). (a) Backscattered electron image. (b) Cu element mapping.
EPMA line analysis results at the cross section of the oxide scales at the scale/steel interface. (a) Base condition (Condition A). (b) Low O2 concentration condition (Condition B).
Under Condition B, an oxidation test was carried out in atmosphere containing H2O but without O2. The EPMA line analysis at the scale/steel interface is shown in Fig. 2(b). The enriched Cu amount at the interface is 0.048 mg cm−2, which is less than that under Condition A. These results indicate that oxidation in atmosphere with a low O2 concentration can decrease Cu enrichment at the scale/steel interface. This implies that reheating in a lower O2 concentration is beneficial for the suppression of hot shortness. There are two reasons why the enriched Cu amount in the case of the oxidation in low O2 concentration atmosphere is less. Firstly, low O2 concentration oxidation reduces the scale thickness. Secondly, some amount of Cu enriched by the oxidation evaporates into the atmosphere, which is discussed below.
3.2. Oxidation-tensile Test ResultsThe surface appearance after the oxidation-tensile test under a wide range of O2 concentration conditions is shown in Fig. 3. There are fewer surface cracks under Conditions D and E compared to under Conditions F and G. Figure 4 shows the surface cross section of the oxidation-tensile test samples. Oxide scales are completely spalled off by the tensile tests. That is why no scales are observed in the pictures. Severe cracks formed under Conditions F and G. Under Condition E, tiny cracks were found. There were no cracks under Condition D. These results indicate that the method of steel reheating in a low O2 concentration atmosphere is effective in suppressing hot shortness.
Surface appearances after the oxidation-tensile tests. (a) Low O2 concentration condition (Condition D). (b) Base condition (Condition E). (c) High O2 concentration condition (Condition F). (d) High O2 concentration condition (Condition G).
Cross sections of the oxide scales after the oxidation-tensile tests. Oxide scales are completely spalled off. (a) Low O2 concentration condition (Condition D). (b) Base condition (Condition E). (c) High O2 concentration condition (Condition F). (d) High O2 condition (Condition G).
The author previously indicated that Cu exists as a solid solution in the upper Fe3O4 layer during scale formation using a TEM analysis.9) Further TEM analysis showed that Cu particles are located along FeO grain boundaries and the liquid Cu was assumed to migrate through the scale from the scale/metal interface and dissolve in the Fe3O4 layer.9) It is necessary to explain two things to indicate this Cu behavior. The first is the reason why liquid Cu precipitates at the scale/steel interface. The second is the reason why liquid Cu migrates through FeO grain boundaries of oxide scale.
When steel containing Cu is oxidized at a high temperature, Cu is precipitated as metal phase at the scale/steel interface. Iron is oxidized to Fe2+ ion at the interface during the oxidation. Cu remains in the steel and enriches at the interface. The solubility of Cu in γphase (fcc) of Fe is about 10 mass% at 1200°C. The partial pressure of oxygen at the interface is considered to be same as that of equilibrium of Fe/FeO. Metallic Cu is stabler than Cu oxide phase at that partial pressure of oxygen. In addition, solubility of Cu into FeO phase is quite low.10) The melting point of metallic Cu is 1083°C. These are the reasons why liquid Cu precipitates at the scale/steel interface during oxidation at high temperature.
Liquid Cu easily migrates through FeO grain boundaries. Takahira et al. reported that liquid Cu has extremely good wettability with reduced Fe from FeO in H2O–H2 containing atmospheres.11) However, They also indicated the good wettability between liquid Cu and solid FeO in H2O–H2 containing atmosphere where FeO is stable.12) Cu has high solubility of Fe and O at a high temperature. Fe is soluble about 6% and O is also soluble about 7% in liquid Cu at 1200°C. Therefore, it is considered that liquid Cu partially dissolves FeO and Cu has good wettability to FeO grains at a high temperature.
In order to understand the behavior of Cu during oxidation with a low O2 concentration, a bell-shaped quartz cover was set over a sample, as shown in Fig. 5(a). A picture of the bell-shaped quartz cover set over the sample oxidized under Condition B is shown in Fig. 5(b). The quartz cover becomes red after the oxidation. The red matter inside the cover was chemically analyzed and Cu and Fe were detected. The XRD on the surface of the oxidized sample under Condition B detected only FeO. On the other hand, a similar oxidation test was carried out in a high O2 concentration atmosphere under Condition C (Fig. 6(a)). After the oxidation, the quartz cover remained transparent (Fig. 6(b)). These results indicate that Cu evaporates through the FeO scale formed in low O2 concentration atmosphere.
The pictures of the bell-shaped quartz cover, which was set over the sample under the low O2 concentration condition (Condition B). (a) Before the experiment. (b)After the experiment.
The pictures of the bell-shaped quartz cover, which was set over the sample under the high O2 concentration condition (Condition C). (a) Before the experiment. (b)After the experiment.
The mass balance of Cu was considered. Figure 7 shows the excluded Cu by oxidation, enriched Cu at the scale/steel interfaces, and Cu in scale. The amount of excluded Cu by oxidation was obtained by mass gain during oxidation, while the amount of evaporated Cu is not shown on the graph because the quartz cover did not catch all of the evaporated Cu. The Cu amounts in the scales were measured by chemical analyses of the oxide scales. The enriched Cu amount at the scale/steel interface was obtained by EPMA analyses. The enriched Cu amount at the interface was less than 20% of excluded Cu by oxidation. In the lower O2 concentration in atmosphere there was less enriched Cu at scale/steel interfaces. One reason for this is that the low O2 concentration atmosphere forms a thin oxide scale. The other is that a greater amount of Cu evaporates in a low O2 atmosphere.
The comparison between excluded Cu by oxidation and detected Cu. The numers shown in this graph mean the ratios to the excluded Cu amount by oxidations.
Partial pressures of gas phases that containing Cu are calculated. Cu2O as solid phase, Cu and CuO as liquid phase, and O2, Cu, Cu2, and CuO as gas phases are taken into account. The equilibrated partial pressures of these phases are calculated. The thermodynamic data are used from the reference.13) Figure 8 shows the relationship between O2 partial pressure and saturated vapor pressure of Cu and Cu oxides. When the liquid Cu equilibrates with Cu gas in an atmosphere of 12%H2O–N2 at 1200°C, the equilibrium partial pressure of the Cu gas is 5.2×10−6 atm. The maximum evaporation flux is expressed in Eq. (1), where p is equilibrium partial pressure, m is mass of molecule, k is Boltzmann’s constant, and T is temperature.
| (1) |
The relationship between O2 partial pressure and saturated vapor pressure of Cu and Cu oxide.
If liquid Cu reaches the scale surface and equilibrates to the atmosphere at the surface at 1200°C, the evaporation flux of Cu is 4.8×10−5 g cm−2 s−1. The evaporation flux is sufficiently great to explain for the missing Cu amount, 0.23 mg cm−2.
Figure 9 indicate Cu behaviors during oxidation of Cu containing steels. In the case of FeO mono layer scale formation under relatively low O2 concentration condition, Cu evaporates through the FeO scale. Liquid Cu is considered to migrate through the grain boundaries of FeO scale, due to the effective wettability between liquid Cu and FeO (Fig. 9(a)). The liquid Cu reaches the scale surface and evaporates as Cu gas. On the other hand, in case of Fe2O3–Fe3O4–FeO three layer scale formation under relatively high O2 concentration condition, liquid Cu migrates through FeO scale grain boundary and then dissolves in Fe3O4 (Fig. 9(b)).
The schematic image of Cu behavior during the oxidation of a steel containing Cu. (a) FeO mono layer scale. (b) Fe2O3–Fe3O4–FeO three layer scale.
This study examined the Cu distribution behavior during the oxidation of steel containing Cu and a method of suppressing surface hot shortness. The following conclusions were drawn:
(1) Cu is not only enriched at the scale/steel interface but is also migrated through scale. The Cu is moved in a Fe3O4 layer in a relatively high O2 concentration. On the other hand, the Cu migrated through the scale and evaporates from the surface as Cu gas when the O2 concentration is relatively low.
(2) The oxidation in a low O2 concentration atmosphere decreases the amount of enriched Cu at the scale/steel interface. It suppresses surface hot shortness.
