METHOD FOR DETERMINING THE PHOSPHORUS CONTENT IN STEEL

Description

[0001] The invention relates to a method for determining the phosphorus content in steel according to the preamble of claim 1.

[0002] Phosphorus is generally undesirable in steel and is often considered a steel pest, as it causes harmful primary segregation during solidification, and secondary segregation can occur in the solidified state. Since it is almost impossible to achieve a homogeneous distribution of phosphorus, the phosphorus content is kept very low, and an upper limit of 0.006%–0.015% is targeted for high-quality steels.

[0003] During steel production, particularly in the LD process (with bottom purging), oxygen must be injected until the phosphorus content has fallen below a required maximum value. At the point when oxygen injection has ended, a sample can be taken, the analysis result of which can provide information about the phosphorus content reached. This could then be used to determine suitable measures such as purging or re-purging to reduce the phosphorus content in the molten steel, if necessary. However, the analysis result arrives too late; it usually takes about five minutes from the time the sample is taken to the time the analysis result is available. For productivity reasons, therefore, waiting for the analysis result is generally not possible. To avoid problems resulting from non-compliance with the phosphorus content, purging or re-purging must be carried out beyond the actually required amount.Therefore, more oxygen is usually injected than necessary, which, however, results in high costs due to the oxygen used and the longer residence time.

[0004] CN103361461 A discloses a method for determining phosphorus content from other measured variables. The phosphorus content is calculated from the bath temperature, the carbon burn-off, the original carbon content of the melt, and the amount of oxygen injected. The measured temperature Tiron is corrected for various influencing factors, such as the amount of scrap (Wscrap), lime (Wlime), and pig iron (Wiron) used, as well as the proportions of phosphorus (w[P]iron) and silicon (w[Si]iron) in the pig iron. The carbon burn-off is determined from the amount of CO and CO2 in the exhaust gas. This model allows for a determination of the phosphorus content within a range of ± 0.003%.

[0005] CN110954670 A discloses a method for continuously predicting the phosphorus content in a converter bath. Starting from the phosphorus content of the pig iron, the change in the bath's phosphorus content is determined for each period of oxygen injection. The parameters for this change include the current bath temperature, the slag reaction, and the iron content in the slag.

[0006] The use of such forecasting methods has so far failed due to the low accuracy of existing calculation methods. In addition, implementation requires significant mathematical effort and the need to record a large number of measured variables and parameters. In particular, when determining changes in phosphorus over a large number of time steps, small errors in the initial parameters can lead to significant deviations in the results and thus unusable forecast values.

[0007] AT406587 B1 discloses a method in which carbon burn-off is used to determine the end of the blowing process in the LD process. However, this method only determines the carbon content of the steel; there is no indication of the applicability of such a method for determining the phosphorus content. Documents CN103160640B and "Development of a new phosphorus partition relation for Australian steelmakers" by Drain Philip and AI disclose additional methods for determining the phosphorus content in steel.

[0008] The object of the invention is to develop a method with which the phosphorus content can be determined with sufficient accuracy at the end of the oxygen injection process, allowing targeted, appropriate measures to reduce the phosphorus content in the steel melt. In addition, a method is to be developed to positively influence the phosphorus content in the melt during the oxygen injection process.

[0009] The invention is based on the task of developing a stable model for determining the phosphorus content with an accuracy of + / - 0.002%, in particular + / - 0.0015%, using a few easily measurable parameters. The primary goal is to use parameters that are already measured in the steel production process.

[0010] The basic idea is to use the carbon burn-off rate during the final phase of oxygen injection into the LD converter, taking into account known influencing factors such as the phosphorus potential in the melt, the slag quantity, and the basicity of the slag, etc., to calculate the phosphorus content in the melt. The carbon burn-off rate is determined from the exhaust gas analyses, particularly those for CO and CO2, and the exhaust gas flow rate. These exhaust gas components, CO and CO2, must be determined very promptly, with a delay of no more than 30 seconds, and in particular, no more than 20 seconds.The method is also based on the use of the equally important influencing factor temperature, which is available very quickly after the end of the oxygen injection with only a slight delay of a maximum of 120 seconds, in particular a maximum of 100 seconds, and thus provides an immediate result which, in combination with the above parameters, now provides a rapid and good forecast of the phosphorus content in the steel melt.The invention thus relates to a method for determining the phosphorus content in a steel bath during the LD process, wherein the bath temperature and the phosphorus content are measured after and / or during the oxygen injection process, characterized in that the course of the phosphorus concentration in the steel is determined by analyzing at least the contents of CO and CO2 in the exhaust gas and the exhaust gas flow, wherein the phosphorus content is determined from the smoothed carbon burnup, the measured temperature of the melt, the amount of phosphorus in the converter from the raw materials used at the start of blowing, the amount of slag used and the basicity of the slag, wherein the calculation is carried out according to the formula. P = f dC dt , T , P sum , Sl , Bas , … is carried out, whereby: [P] ... phosphorus concentration in the steel, dC / dt ... C burn-up, T ... temperature of the melt, Psum ... P quantity in the converter, Sl... slag quantity, Bas ... basicity of the slag.

[0011] A further development provides that the temperature is included in the calculation inversely exponentially using the following formula: P = f dC dt , e − k T , P sum , Sl , Bas , …

[0012] A further development proposes that, to improve the forecast accuracy, the carbon burn-off rate is taken into account using a logarithmic approach, where: P = f ln dC dt , T , P sum , Sl , Bas , … or P = f ln dC dt , e − k T , P sum , Sl , Bas , … where the constant k is determined from records of different batches in which the phosphorus content of the finished steel is measured by chemical analysis.

[0013] Further training provides that oxygen activity is taken into account as follows: P = f dC dt , e − k T , P sum , Sl , Bas , O 2 akt ,   …

[0014] According to the present invention, the phosphorus content is evaluated based on the following formula: P = k 1 ⋅ ln dC dt + k 2 ⋅ e − k 3 T + k 4 ⋅ P sum Sl + k 5 ⋅ Bas + const or P = k 1 ⋅ ln dC dt + k 2 ⋅ e − k 3 + k 4 dC dt T + k 5 ⋅ P sum Sl + k 6 ⋅ Bas + const where the parameters k1 -k6 and const be determined from the records of various batches in which the phosphorus content of the finished steel was measured by chemical analysis.

[0015] A further development provides that the analysis values ​​from the exhaust gas analysis, in particular the values ​​of CO and CO2, are included in the calculation with a maximum delay of 30s, in particular 20s.

[0016] A further development provides for the carbon burn-off to be included in the calculation directly or logarithmically.

[0017] A further development provides for the phosphorus content of the finished steel melt to be measured by chemical analysis, and the parameters of the calculation to be continuously adjusted using these values, in particular using the Gauss method.

[0018] A further development provides that the measured oxygen activity is included in the calculation.

[0019] The invention is explained by way of example with reference to a drawing. It shows: Figure 1: A diagram showing the typical evolution of carbon and phosphorus during blowing; Figure 2: A diagram showing the comparison of the calculated values ​​with measured values.

[0020] In the final phase of oxygen injection (after approximately 95% of the injection time), phosphorus can be reduced again due to a lack of carbon. This leads to a carbon deficiency after the temperature rise and the resulting dominance of the reaction 2C+O2 ->2CO over P2+5O->P2O5, which led to a reduction of P2O5 and thus an increase in phosphorus in the melt. Therefore, a relationship arises between carbon content, carbon burn-off, and phosphorus content in the final phase of oxygen injection.

[0021] Experiments have shown that the formula P = k 1 ⋅ ln dC dt + k 2 ⋅ e − k 3 T + k 4 ⋅ P sum Sl + k 4 ⋅ Bas + const determined values ​​( Figure 2 , Y-axis) a high agreement with the measured values ​​( Figure 2 , X-axis).

[0022] In concrete terms, this means that at the time of temperature measurement, a useful estimate of the phosphorus content of the current melt is now available. As already mentioned, this results in a time advantage of approximately 5 minutes until any sample results are available. Using the same calculation formula and the target temperature for the process, the time-varying phosphorus content of the melt can be calculated while oxygen is being injected. This, in turn, allows the oxygen injection process to be extended in a targeted manner if the calculation shows that the desired phosphorus content will be exceeded if the process is terminated solely based on the carbon content.

Claims

1. Method to determine the phosphorus content in a steel bath during the LD process, wherein, after and / or during the process of blowing in oxygen, the bath temperature and the proportion of phosphorus is measured, characterised in that the development of the phosphorus concentration in the steel is determined by analysis of at least the content of CO and CO2 in the exhaust gas and the exhaust gas flow, wherein the phosphorus content is determined from the smoothed carbon burn-up, the measured temperature of the melt, the amount of phosphorus in the converter from the added raw materials at the start of the blowing, the added amount of slag and the basicity of the slag, wherein the calculation is performed according to the formula, P = f dC dt , T , P sum , Sl , Bas , … where: [P ]... Concentration of phosphorus in the steel, dC dt ... C-burn-up, T ... Temperature of the melt, Psum ... P-amount in the converter, Sl ... Amount of slag, Bas ... Basicity of the slag, wherein the phosphorus content is calculated using the following formula: P = k 1 ⋅ ln dC dt + k 2 ⋅ e − k 3 T + k 4 ⋅ P sum Sl + k 5 ⋅ Bas + const or P = k 1 ⋅ ln dC dt + k 2 ⋅ e − k 3 + k 4 dC dt T + k 5 ⋅ P sum Sl + k 6 ⋅ Bas + const Wherein the parameters k1-k6 and const are determined from records of different batches, for which the phosphorus content of the finished steel was measured by chemical analysis, wherein the phosphorus content of the finished steel melt is measured by chemical analysis, and the parameters of the calculation are iteratively adapted with these values using the Gauss method.

2. Method according to Claim 1, characterised in that the temperature is included in the calculation as an inverse of an exponential function using the following formula: P = f dC dt , e − k T , P sum , Sl , Bas , … 3. Method according to one of the preceding claims, characterised in that to improve the prediction accuracy the carbon burn-up rate is included using a logarithmic approach, wherein: P = f ln dC dt , T , P sum , Sl , Bas , … or P = f ln dC dt , e − k T , P sum , Sl , Bas , … wherein the constant k is determined from records of different batches, for which the phosphorus content of the finished steel is measured by chemical analysis.

4. Method according to one of the preceding claims, characterised in that the oxygen activity is measured and factored as follows: P = f dC dt , e − k T , P sum , Sl , Bas , O 2 akt ,   … , wherein O2akt is the measured oxygen activity.

5. Method according to one of the preceding claims, characterised in that the measured temperature is included in the calculation as an inverse of an exponential function.

6. Method according to Claim 1, characterised in that the analytical values from the exhaust gas analysis, in particular the values of CO and CO2 are included in the calculation with a maximum delay of 30s, in particular 20s.

7. Method according to Claim 3, characterised in that the carbon burn-up is included in the calculation directly or logarithmically.

8. Method according to one of the preceding claims, characterised in that the measured oxygen activity is included in the calculation.