## 3 Results and Discussion
### Verification of the Consistency of the Decarburization Curve  
We compared the decarburization curves obtained using Equation (2) with the actual carbon concentrations measured through sampling in the molten steel (Figure 4). Three of the 12 operations used in the comparison were the cases in which carbon was added to the molten steel during the RH operation. We calculated the decarburization curves by using the measured values of the TDLAS and NDIR analyzer, respectively. In the decarburization curves using the TDLAS analyzer, it was possible to confirm the point at which the decarburization rate changed rapidly. This is consistent with the results of previous studies [14; 17; 20] dividing the operation stage into two to three stages based on the period when the decarburization reaction changes rapidly in RH operation. On the other hand, it was difficult to clearly distinguish the stages of the decarburization reaction in the decarburization curve using the NDIR analyzer. When carbon was not added during operation, Figure 4a shows that the decarburization curve using the TDLAS analyzer was similar to the actual amount of dissolved carbon, but the decarburization curve using the NDIR analyzer did not reflect the change in the actual values. From the results of Figure 3, we can deduce why the decarburization curve using the NDIR analyzer is different from the actual carbon concentration. It is difficult for the NDIR analyzer to immediately detect changes in the off-gas components emitted from the molten steel even if the decarburization reaction occurs intensely due to the installation location and its characteristics. In addition, even when the decarburization reaction decreases rapidly, the measured value of carbon oxides t  
Figure 4: Comparison of the decarburization curves and the actual carbon concentrations (filled circles). Operations (**a**) without or (**b**) with additional carbon. The solid and dotted lines represent the decarburization curves obtained using TDLAS and NDIR analyzers, respectively. slowly. Therefore, the decarburization curves obtained using the NDIR analyzer changed more slowly than the actual amount of dissolved carbon and showed relatively larger values than the actual values when the decarburization reaction was slow. When carbon was added during the RH operations, Figure 4b showed that the decarburization curves differed from the actual amount of dissolved carbon. Additional carbon had been added about 3-4 min after the start of operation and the dissolved carbon content measured at about 6 min had increased. The difference in values was reasonable because we did not consider the effect of added carbon when calculating the decarburization curve. It is a meaningful task to identify changes in the decarburization curve due to the addition of carbon, but it is beyond the scope of this study. Instead, we used the weight of the added carbon as an input to the ANN model to train the effect of added carbon on endpoint carbon concentration. Figure 5 shows the error between the predicted value of the decarburization curve using the TDLAS analyzer and the measured value of the carbon concentration in the molten steel for all the sampling values. At the early stage of the operations, the errors between the predicted value and the actual value were very large, but decreased rapidly as time passed. One of the reasons for the large prediction error in the early stages of the operations may be the difference between the sampling time of the molten steel and the recording time of the operation data. Since the decarburization reaction occurs very intensely in the early stages of operation, the carbon concentration may change significantly within a short time. Considering the manual sampling of the molten steel, even a small difference between the sampling time and the data recording time may increase the prediction error. In addition, the measured carbon concentration may vary depending on the sampling location because the molten steel is not uniformly mixed during this period [23; 24; 53].