EP4112750A1 - Method for determining phosphorus content in steel - Google Patents

Abstract

The invention relates to a method for determining the phosphorus content in a steel bath during the LD process, the bath temperature and the phosphorus content being measured after and/or during the process of blowing in oxygen, characterized in that the course of the phosphorus concentration in the steel is determined by the analysis at least the CO and CO2 content in the flue gas and the flue gas flow is determined, with the phosphorus content being determined from the smoothed carbon burn-off, 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 calculated according to the formula P=fdCdt,T,Psum,Sl,Bas,... where:[P] ...concentration of phosphorus in steel,dCdt ...C burn-off,T... Melt temperature,P_sum ...P quantity in the converter,Sl ...slag quantity,Bas ...basicity of the slag.

Description

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

Phosphorus is usually undesirable in steel and is often regarded as a steel pest, since phosphorus causes harmful primary segregation during solidification, and secondary segregation can occur in the solidified state. Since it is hardly possible to achieve a homogeneous distribution of the phosphorus, the phosphorus content is kept very low and an upper limit of 0.006%-0.015% is aimed at for high-quality steels.

In the production of steel, especially in the LD process (with bottom flushing), oxygen must be blown in until the phosphorus content has fallen below a required maximum value. A sample can be taken when the oxygen injection is complete, the analysis result of which can provide information about the phosphorus content achieved. Suitable measures such as rinsing or after-blowing to reduce the phosphorus content in the molten steel, if necessary, could be derived from this. However, the analysis result comes too late, it usually takes about 5 minutes from the time the sample is taken until the analysis result arrives, so waiting for the analysis result is usually not an option for reasons of productivity. In order to avoid problems arising from non-compliance with the phosphorus content, the level of rinsing or blowing that is actually required must be exceeded. For this reason, more oxygen is usually blown in than necessary, which, however, results in high costs due to the oxygen used and the longer residence time.

From the CN103361461A a method for determining the phosphorus content from other measured variables is known, with the phosphorus content being calculated from the bath temperature, the carbon burnup, the original carbon content of the melt and the amount of oxygen blown in. The measured Tiron temperature is adjusted by 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) corrected in pig iron. The carbon burnup is determined from the amount of CO and CO2 in the exhaust gas. With this model it is possible to determine the phosphorus content in the range of ± 0.003%.

From the CN110954670A a method for the continuous prediction of the phosphorus content in a converter bath is known. Based on the phosphorus content of the pig iron, the change in the phosphorus content of the bath is determined for each time period in which oxygen is blown in. The current bath temperature, the slag reaction and the proportion of iron in the slag go into this change as parameters.

The use of such prognosis methods has so far failed due to the accuracy of the prediction of known calculation methods. In addition, a high mathematical effort is necessary for the implementation and the need to record a large number of measured variables and parameters. In particular, when determining a change in the phosphorus over a large number of time steps, small errors in the start parameters can lead to large deviations in the results and therefore unusable prognosis values.

From the AT406587 B1 a method is known in which the carbon burnup is used to determine the end of the blowing process in the LD process. However, only the carbon content of the steel is inferred; there is no indication that such a method can be used to determine the phosphorus content.

The object of the invention is the development of a method with which the phosphorus content can already be determined with sufficient accuracy at the end of the blowing in of oxygen in order to be able to take specific suitable measures to reduce the phosphorus content in the molten steel. In addition, a method is to be developed to positively influence the phosphorus content in the melt during the oxygen injection process.

The object of the invention is to develop a stable model for determining the phosphorus content with an accuracy of +/-0.002%, in particular +/-0.0015%, with a small number of easily ascertainable measured variables. In doing so, reference should primarily be made to measured variables that are already recorded in the steelmaking process anyway.

The basic idea is to use the carbon burn-off rate during the final phase of blowing oxygen into the LD converter, also taking into account known influencing variables such as the phosphorus potential in the melt, the amount of slag and the basicity of the slag, etc. to calculate the phosphorus content in the melt. The carbon burn rate is determined from the exhaust gas analyses, in particular those for CO and CO2, and the exhaust gas flow, with these exhaust gas components CO and CO2 having to be determined very promptly, with a maximum delay of 30s, in particular a maximum of 20s. The method is also based on using the also very important influencing variable temperature, which is available very soon after the end of blowing in oxygen 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 quick and good forecast of the phosphorus content in the molten steel.

erfolgt, wobei dabei ist:

• [P ] ... Konzentration Phosphor im Stahl,
• dC/dt ... C-Abbrand,
• T ... Temperatur der Schmelze,
• Psum ... P-Menge im Konverter,
• Sl ... Schlackenmenge,
• Bas ... Basizität der Schlacke.

The invention thus relates to a method for determining the phosphorus content in a steel bath during the LD process, the bath temperature and the phosphorus content being measured after and/or during the oxygen blowing-in process, characterized in that the course of the phosphorus concentration in the steel is determined by the analysis at least the CO and CO2 content in the flue gas and the flue gas flow is determined, with the phosphorus content being determined from the smoothed carbon burn-off, 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 is determined, the calculation according to the formula $$\left[P\right]=f\left(\frac{\mathit{DC}}{\mathit{German}},T,P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

takes place, where:

• [P ] ... concentration of phosphorus in the steel,
• dC/dt ... C burn-up,
• T ... temperature of the melt,
• Psum ... P quantity in the converter,
• Sl ... amount of slag,
• Bas ... basicity of the slag.

A further development provides that the temperature is used inversely exponentially in the calculation using the following formula: $$\left[P\right]=f\left(\frac{\mathit{DC}}{\mathit{German}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

oder $$\left[P\right]=f\left(\mathit{ln}\frac{\mathit{dC}}{\mathit{dt}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{Sl},\mathit{Bas},\dots \right)$$

wobei die Konstante k aus Aufzeichnungen verschiedener Chargen ermittelt wird, bei denen der Phosphorgehalt des fertigen Stahls mit chemischer Analyse gemessen wird.A further development provides that, in order to improve the accuracy of the prediction, the carbon burn-off rate is entered using a logarithmic approach, where: $$\left[P\right]=f\left(\mathit{ln}\frac{\mathit{DC}}{\mathit{German}},T,P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

or $$\left[P\right]=f\left(\mathit{ln}\frac{\mathit{DC}}{\mathit{German}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

where the constant k is determined from records of various heats where the phosphorus content of the finished steel is measured by chemical analysis.

A further development provides that the oxygen activity is taken into account as follows: $$\left[P\right]=f\left(\frac{\mathit{DC}}{\mathit{German}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{SL},\mathit{base},\mathrm{<figure-callout\; id="2"\; label="O"\; filenames="imgf0001.png"\; state="\{\{state\}\}">O</figure-callout>}{2}_{\mathit{act}},...\right)$$

A further development provides that the phosphorus content is calculated using the following formula: $$\left[P\right]=k_1\cdot \ln \frac{\mathit{DC}}{\mathit{German}}+{\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}_2\cdot e^{-\frac{k_3}{T}}+k_4\cdot \frac{P_{\mathit{sum}}}{\mathit{SL}+k_5\cdot \mathit{base}}+\mathit{const}$$

wobei die Parameter k₁-k₆ und const aus den Aufzeichnungen verschiedener Chargen ermittelt werden, bei denen der Phosphorgehalt des fertigen Stahls mit chemischer Analyse gemessen wurde.A further development provides that the phosphorus content is calculated using the following formula: $$\left[P\right]=k_1\cdot \ln \frac{\mathit{DC}}{\mathit{German}}+{\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}_2\cdot e^{-\frac{k_3+k_4\frac{\mathit{DC}}{\mathit{German}}}{T}}+k_5\cdot \frac{P_{\mathit{sum}}}{\mathit{SL}+{\mathrm{<figure-callout\; id="6"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}_6\cdot \mathit{base}}+\mathit{const}$$

where the parameters k ₁ -k ₆ and const are determined from the records of different batches where the phosphorus content of the finished steel was measured with chemical analysis.

A 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 30 s, in particular 20 s.

A further development provides that the carbon burnup is included in the calculation directly or logarithmically.

A development provides that the phosphorus content of the finished molten steel is measured using a chemical analysis, and the parameters of the calculation are continuously adjusted using these values, in particular using the Gauss method.

A development provides that the measured oxygen activity is included in the calculation.

Figur 1:

ein Diagramm zeigend den typischen Verlauf von Kohlenstoff und Phosphor während des Blasens;

Figur 2:

Ein Diagramm zeigend den Vergleich der berechneten Werte mit gemessenen Werten.

The invention is explained by way of example using a drawing. They show:

Figure 1:

a diagram showing the typical evolution of carbon and phosphorus during blowing;

Figure 2:

A diagram showing the comparison of calculated values with measured values.

In the final phase of blowing in oxygen (after approx. 95% of the blowing time), phosphorus can, after it has meanwhile at approx. 90% blowing time due to the temperature rise and the resulting dominance of the reaction 2C+O ₂ ->2CO compared to P ₂ +5O- >P ₂ O ₅ has led to the reduction of P ₂ O ₅ and thus to an increase in phosphorus in the melt can now be broken down again due to the lack of carbon. There is therefore a relationship between carbon content, carbon burn-off and phosphorus content in the final phase of oxygen injection.

ermittelten Werte (Abbildung 2, Y-Achse) eine hohe Übereinstimmung mit den Messwerten (Abbildung 2, X-Achse) haben.Experiments have shown that with the formula $\left[P\right]=k_1\cdot \ln \frac{\mathit{DC}}{\mathit{German}}+k_2\cdot e^{-\frac{k_3}{T}}+k_4\cdot \frac{P_{\mathit{sum}}}{\mathit{SL}+k_4\cdot \mathit{base}}+\mathit{const}$

determined values ( Figure 2 , Y-axis) a high agreement with the measured values ( Figure 2 , X-axis).

In concrete terms, this means that at the time the temperature is measured, there is now also a usable estimate of the phosphorus content for the current melt. As already mentioned, this brings a time advantage of approx. 5 minutes until the arrival of a possible sample result. With the same calculation formula and using the target temperature for the process, the temporally variable phosphorus content of the melt can already be calculated during the injection of oxygen. This in turn means that the oxygen injection process can be extended in a targeted manner if the calculation shows that the desired phosphorus content is exceeded if the end of the process is only based on the carbon content.

Claims (11)

A method for determining the phosphorus content in a steel bath during the LD process, the bath temperature and the phosphorus content being measured after and/or during the oxygen blowing-in 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 is determined in the exhaust gas and the exhaust gas flow, with the phosphorus content being determined from the smoothed carbon loss, 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, with the Calculation according to the formula $$\left[P\right]=f\left(\frac{\mathit{DC}}{\mathit{German}},T,P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

takes place, where:
[ P ] ... concentration of phosphorus in the steel, $\frac{\mathit{DC}}{\mathit{German}}$

... C burn-up,
T... temperature of the melt,
P _sum ... P quantity in the converter,
S /... amount of slag,
Bas ... basicity of the slag. Method according to Claim 1, characterized in that the temperature is used inversely exponentially in the calculation using the following formula: $$\left[P\right]=f\left(\frac{\mathit{DC}}{\mathit{German}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

Method according to one of the preceding claims, characterized in that to improve the prediction accuracy, the carbon burn rate is entered with a logarithmic approach, where: $$\left[P\right]=f\left(\mathit{ln}\frac{\mathit{DC}}{\mathit{German}},T,P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

or $$\left[P\right]=f\left(\mathit{ln}\frac{\mathit{DC}}{\mathit{German}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{SL},\mathit{base},...\right)$$

where the constant k is determined from records of various heats where the phosphorus content of the finished steel is measured by chemical analysis. Method according to one of the preceding claims, characterized in that the oxygen activity is taken into account as follows: $$\left[P\right]=f\left(\frac{\mathit{DC}}{\mathit{German}},e^{-\frac{k}{T}},P_{\mathit{sum}},\mathit{SL},\mathit{base},O{2}_{\mathit{act}},...\right)$$

Method according to one of the preceding claims, characterized in that the phosphorus content is calculated using the following formula: $$\left[P\right]=k_1\cdot \ln \frac{\mathit{DC}}{\mathit{German}}+k_2\cdot e^{-\frac{k_3}{T}}+k_4\cdot \frac{P_{\mathit{sum}}}{\mathit{SL}+k_5\cdot \mathit{base}}+\mathit{const}$$

Method according to one of the preceding claims, characterized in that the phosphorus content is calculated using the following formula: $$\left[P\right]=k_1\cdot \ln \frac{\mathit{DC}}{\mathit{German}}+k_2\cdot e^{-\frac{k_3+k_4\frac{\mathit{DC}}{\mathit{German}}}{T}}+k_5\cdot \frac{P_{\mathit{sum}}}{\mathit{SL}+k_6\cdot \mathit{base}}+\mathit{const}$$

where the parameters k ₁ -k ₆ and const are determined from records of different batches where the phosphorus content of the finished steel was measured by chemical analysis. Method according to one of the preceding claims, characterized in that the measured temperature is included in the calculation inversely exponentially. Method according to one of the preceding claims, characterized in 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 30 s, in particular 20 s. Method according to one of the preceding claims, characterized in that the carbon burn-off enters the calculation directly or logarithmically. Method according to one of the preceding claims, characterized in that the phosphorus content of the finished molten steel is measured using a chemical analysis, and the parameters of the calculation are continuously adjusted using these values, in particular using the Gauss method. Method according to one of the preceding claims, characterized in that the measured oxygen activity is included in the calculation.

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