introduction
In the continuous casting process, measures must be taken to avoid the contact between the molten steel and the atmosphere. Therefore, when the molten steel is transported from the ladle to the tundish, a long shroud is usually used between the ladle and the tundish to provide a safe air-shielding channel for the molten steel. Despite these innovations (such as improved shroud design, automatic installation of shrouds, sealing the junction between shroud and tundish shroud with ceramic gaskets, etc.), the exposure of molten steel to the atmosphere cannot be completely avoided, especially when the molten steel passes through the long shroud. Nozzle or conventional tundish during the first furnace molten steel injection. When the molten steel flows out of the long shroud, it takes several minutes to flow through the tundish environment as an unconstrained liquid jet, until the port below the long shroud is submerged by the molten steel in the tundish pool before it reaches the normal casting state. As a result, the nitrogen and oxygen content in the molten steel in the tundish at this time increases, and these eventually manifest in the solidified slab, causing the chemical composition of the slab to fluctuate beyond the contractual agreement. In order to quantify and evaluate the severity of tundish molten steel exposure to the atmosphere and the concomitant compositional changes, the industry often detects the total nitrogen and total oxygen content of billets or slabs by chemical analysis.
The problems of nitrogen and oxygen absorption in molten steel during molten steel casting have been extensively studied in steel mill production and laboratory environments. As far as continuous casting is concerned, the vast majority of studies are mainly limited to the study of [N] and [O] uptake by the first slab or billet after casting. Few studies have been done on the secondary oxidation of tundish molten steel in contact with the atmosphere. However, little has been noticed that as the length of the cast billet or slab increases, the effects of nitrogen and oxygen uptake with the length of the slab are different.
Therefore, the purpose of this study was to study the exposure of molten steel to the atmosphere during the start of casting in the long shroud channel or tundish, and the consequent nitrogen uptake of the molten steel. As a result, a method was developed to predict the length of strands that absorb nitrogen during continuous casting.
For this purpose, a physics-based modeling approach is proposed below. Measurements to support the mill site are also included to confirm the current findings.
Work now
During the first molten steel casting process in the tundish, the nitrogen (and oxygen) in the tundish atmosphere is transferred to the molten steel, which increases the nitrogen content in the molten steel. In the tundish, the nitrogen absorption completely stops only when the end of the shroud is immersed in the molten pool. This means that when the molten steel initially enters the tundish, it is mixed with the atmosphere and has a significantly different chemical composition, and then the molten steel level continues to rise in the tundish until the outlet end of the shroud is submerged. Since the composition (i.e., in terms of [N], [O], etc.) of the molten steel entering from the ladle differs from the composition of the molten steel present in the tundish, Fig. 1 schematically depicts the division of the first heat of the tundish into two stages, It is divided and indicated by the time when the outlet end of the shroud is submerged by the molten steel in the tundish. In the second stage, the composition of molten steel in the tundish gradually changes, and the weight and nitrogen content of the molten steel in the tundish gradually reach a stable state. As we will notice, it is essentially the butt of different steel grades during the continuous casting operation. Mix the same. The change of molten steel composition at the beginning of the first heat of molten steel in the tundish must be accompanied by the fluctuation of the composition change. Therefore, it is necessary to take countermeasures to study the change and mixing phenomenon of the composition of the molten steel received by the tundish in the first ladle.
（Figure 1 conceptually divides the initial casting of molten steel in the first heat of the tundish into two stages. Taking the submerged molten steel at the outlet of the long shroud as the dividing point, the curve relationship of the nitrogen content of molten steel in the tundish with time is illustrated.(t0 = the moment when the first ladle of molten steel in the tundish is poured; t1 = the tapping end of the long nozzle is submerged by the molten pool of molten steel in the tundish. t2 = constitutes a stable casting state.The figure is divided into two stages, stage I is the period when the long shroud is flooded with molten steel; stage II is the period when the long shroud is submerged until the nitrogen content is stable. )
Therefore, in order to determine the volume of molten steel in the tundish exposed to the atmospheric environment with a change in chemical composition (ie [N] is higher) and the corresponding length of the bloom after solidification. Mixed simulation experiments were carried out in a full-scale water model of a two-strand tundish system.
In a typical experiment, the model tundish is first injected with brine, the brine has a fixed uniform conductivity, and the tundish brine liquid level is up to the outlet end of the shroud. Then, regular water enters the simulated tundish through the long shroud at a prescribed flow rate. The two outlets of the tundish were opened at about the same time to simulate the pouring of steel, and the conductivity of the flowing water was continuously monitored by EUTECHTM conductivity probes placed near the two outlets of the tundish. The conductivity versus time response curve thus obtained essentially determines the time asymptotic process of this change in liquid conductivity over time and the continuation of mixing in the tundish. In a typical experiment, the sequence of operations is shown in Figure 2, while the physical dimensions and operating parameters relevant to this study are summarized in Table 1.
Fig. 2 In order to study the change of molten steel composition near the outlet of the tundish, in a typical simulation experiment, a schematic diagram of three different stages of the sequence of filling the tundish with common water and monitoring the conductivity of the outlet.
Stage 1: Use salt water to enter the tundish until the outlet end of the shroud is submerged;
Stage 2: gradually fill the common water into the tundish, and detect the conductivity at the outlet of the tundish;
Stage 3: Continuously measure the conductivity of the mixed water at the outlet until the brine is completely replaced and a steady state of operation is reached. )
parameter | steel mill tundish | Model |
Bottom length of tundish, mm | 1700 | 1700 |
The maximum width of the bottom of the tundish, mm | 600 | 600 |
The number of egress flows in the tundish | 2 | 2 |
Tundish nozzle diameter, mm | 40 | 40 |
Inner diameter of long nozzle, mm | 67 | 70 |
Tundish liquid depth under stable operating conditions, mm | 670-700 | 700 |
Distance between the outlet end of the shroud and the bottom of the tundish, mm | 500-550 | 535 |
Liquid volume of tundish, submerged from the liquid level to the fracture of the long nozzle, kg | ~4500 | 612.5 |
Casting rate, kg/min | 600-700 | 100 |
Estimated range of fluid exposure time in the tundish, s | 385-450 | 367.5 |
Mold cavity size: mm × mm | 200*200 |
|
Casting speed: m/s | 0.015-0.0158 |
|
Slab length, m | 4.1 |
|
(Annotation: The slab length 4.1m in the table is estimated to be a typo, because the slabs mentioned later are all 4.5m.)
With regard to conductivity measurements, it is necessary to mention here that several experiments were carried out under a given set of operating conditions to assess the reproducibility, thus showing that successive measurements of conductivity over time are all closely related , and the conductivity changes recorded at the two symmetrically arranged tundish outlets are virtually identical. Experimental measurements are always made at a constant tundish inlet and outlet flow, by raising or lowering the stopper to achieve a given pull rate, or lowering the tundish liquid depth to change the outlet flow. The experiments also investigated the effects of the initial state, motion and rest states of the tundish melt pool on the instantaneous conductivity. The marginal change in instantaneous conductivity results from such repeated experiments.
Since the scale factor between the steel mill tundish system and the model is unified, since the volume of liquid entering and leaving the tundish is constant, the following volume conservation equation applies:
In formula (1), Q is the volume per unit time, A is the cross-sectional area, and U is the flow rate. The suffix is self-explanatory and needs no further explanation. Since the model is geometrically identical to the actual tundish, the fluid dynamics are similar, and therefore the kinematics are similar. [11] Therefore, the model is on the same time scale as the actual tundish system of the steel plant and is related to the continuous casting parameters and the experimental parameters, according to:
The casting length L cast(t) (= t × U cast; U cast is the casting speed, constant) is uniquely related to the corresponding time in the model tmod, according to:
Therefore, given the length of each billet (usually all the billets produced by a steel mill are of the same length), the total cast length estimated from equation (3) can be calculated as the cumulative sum of the lengths of billets produced. Therefore, the changes in conductivity/nitrogen content during the experiment can be explained both by the time of the experiment and by the length of a single slab, as shown in Fig. 3. Here, it can be clearly seen that the conductivity (ie salt concentration) of the tundish effluent water is very high for the first 500 seconds or so. The conductivity drops rapidly between 500 and 800 seconds and almost completely disappears around 1200 seconds. The total duration, as noted here, is almost four strands from the time the long shroud is flooded by the tundish to reach the nitrogen content stabilization target. Extrapolating the results of Figure 3 to the field operation of the steel mill, it can be seen that the continuous casting process is in progress. The molten steel in the first heat was exposed to the tundish, which caused the molten steel to absorb nitrogen, which was likely to be manifested in the inspection of the first three billets, while the fluctuation of nitrogen content almost completely disappeared after the fifth billet.
（Figure 3. Change in conductivity of water measured near the tundish outlet, as a function of time, corresponding to the number and length of strands produced by the steel mill’s continuous caster）
To further confirm this finding, the nitrogen content of the first five strands produced from the first heat of the SCr420HB tundish was measured at a steel mill using a LECO™ Oxygen and Nitrogen Meter. Under the same continuous casting conditions, a total of 11 experiments were carried out, and the nitrogen content of the tail ends of the five head billets was measured. The data is summarized in Table 2. On this basis, the nitrogen content of each strand was estimated. And the degree of increase in nitrogen content of the slabs in different sections (compared with the nitrogen content [N] measured by sampling after the VD is opened). These data are presented in Table 2. Qualitatively, the variation trend of nitrogen content in the first five strands of molten steel in the first tundish is very similar to the variation trend of electrical conductivity observed in the physical model (see Figure 3).
Table 2 Comparison of nitrogen content data of the first five billets in the first molten steel continuous casting process of SC420HB steel tundish
In order to visually compare the physical model and the measurement results of the industrial production continuous caster of the steel mill from a quantitative point of view, the nitrogen content in the in-situ slab shown in Table 2 is dimensionless, as follows:
Referring to formula (4), it is important to mention here that, in order to eliminate the effect of suction when molten steel passes through the long nozzle (although it is small, suction is still common), the effect of this nitrogen increase is removed from the current analysis, Therefore, to fully quantify the change in nitrogen content due to the initial stage when the tundish begins to receive the molten steel of the first heat, the molten steel is exposed to the atmosphere of the tundish, using the above dimensionless definition instead of [N]bloom/[N]bloom,1.
Similarly, the instantaneous conductivity values in Fig. 3 are also dimensionless processed according to (instantaneous conductivity-final conductivity)/(initial conductivity-final conductivity). In Figure 4, the slab value, the dimensionless nitrogen content is directly superimposed corresponding to the experimentally measured dimensionless conductivity, which is derived from Figure 3. As seen in Figure 4, a very reasonable agreement between the physical model and the nitrogen content measurements at the steel mill site is evident. This is to be expected because in geometrically and kinetically similar non-reactive systems, the dimensionless concentration distributions always correspond.
Fig. 4 Comparison of dimensionless nitrogen content of the first five casting slabs of the first molten steel in the tundish and the corresponding dimensionless conductivity derived from the full-scale water die tundish (the experimental conductivity curve is the difference between the two response curves in Fig. 3 average value)
Finally, in the initial stage of molten steel filling in the first heat of the tundish, the molten steel exposed to the non-standard tundish environment will increase the nitrogen content, which is obviously manifested in the length direction of the slab. By reducing the exposure time of molten steel jets into the tundish atmosphere, as this study seems to show, increasing the shroud length can minimize the length of the strands where quality problems arise from increased nitrogen content. However, implementing any operational changes at the level of mature large-scale industrial production is difficult. Because any increase in the length of the long shroud amplifies the splash of molten steel at the beginning of the first tundish pouring, or causes the refractory material in the impact zone to be severely eroded and the temperature rises significantly. Nonetheless, optimizing the shroud length may be beneficial in the current environment, especially in suboptimal operating environments. In addition, the pre-filling of the tundish with inert gas also reduces nitrogen (and oxygen) absorption, which in turn reduces the length of the cast strand affected by composition.
Conclusion
During the initial stage of pouring, changes in molten steel composition due to exposure to the tundish environment were studied in a full-scale water model, including the long shroud mechanism. The experimental results show that, in the typical casting process, the composition changes in a long time, which mainly affects the composition of the first three billets in the first heat of the tundish. The observations from the physical model also agree with the measurements of the actual caster of the steel mill.
