EP0063569B1 - Method for the production of steel in a basic iconverter using slag - Google Patents

Abstract

By the present method, the basicity of the starting slag is significantly increased and the slag is kept at the saturation threshold, if possible during all the refining, but at least during the critical starting period. It is desirable to reach the MgO saturation as well as the slag saturation with CaO and calcium disilicate. At the end of the blowing, the method is carried out with a constant amount of slag per ton of steel for a given Si contents of the pig iron. A certain amount of MgO per ton of steel, according the Si contents of the pig iron, is added to the starting cast-iron together with the slagging medium, before or during the start of the refining; after the start of the refining or after the melt, the totality of the slag stays in the converter. This results, on the one hand, in the fact that the lifetime of the oven's shin is prolonged and, on the other hand, in the fact that the quantity of slagging medium required is significantly reduced. The slag adjustment is more regular and the metallurgical results are improved.

Description

The present invention relates to a method for producing steel in a basic converter using liquid converter slag, in which an end slag saturated with MgO and CaO or dicalcium silicate is obtained.

For basic brick lining of converters, fired or tar-bonded stone types based on dolomitic (MgO + CaO) or magnesitic (MgO) as well as combinations of both are used. These bricks are subject to strong chemical, thermal and mechanical wear during the fresh process. Mechanical wear and tear is caused by erosion of the feed materials, by the use of scrap and pig iron. Thermal wear arises from the sensitivity of the refractory bricks to temperature fluctuations. However, the durability of the converter lining is mainly influenced by chemical causes, i.e. through the attack of the slag on the basic lining.

It is therefore the aim of the present invention, inter alia, to increase the durability of basic bricks by reducing chemical wear, by means of special operational measures when carrying out the fresh process. As is known, chemical wear depends on the composition of the slag during the fresh process. According to the current state of knowledge, a high iron oxide content has a negative influence, especially at the prevailing high temperatures, since a reaction takes place between the iron oxide and the carbon of the refractory delivery binder, which is decarburized in the first phase of wear. It is also known that increasing silicon contents reduce the service life of the basic refractory materials. But especially in the first minutes of the freshening process, due to the silicon content of the pig iron and the corresponding iron oxide, high silicon oxide concentrations and an acidic slag form, since the lime used cannot dissolve so quickly.

Because low-viscosity low-basicity iron oxide-silicate slags behave aggressively towards the basic brick lining, they infiltrate into the stone pores during the blowing process and react there with the CaO of the dolomite. In addition, this slag has a considerable solvency for MgO. This solubility of the slag is greatest at the beginning of the freshening process and decreases at the end of the blowing with increasing basicity. The slag attack on the converter lining is the greatest at the beginning of the fresh season.

Efforts to increase shelf life must therefore go in the direction of increasing flaccidity, especially in the early stages of freshening. A chemical-metallurgical attack of the slag on the refractory, basic converter lining can only be reduced by saturating the slag with MgO and CaO or dicalcium silicate during the entire fresh process. The composition of the final slag alone cannot be used as a criterion for the wear of the refractory material. However, saturation of the initial slag with MgO and CaO has so far encountered great difficulties because the introduced MgO and CaO do not dissolve sufficiently quickly. Solubility only improves during the fresh process.

In DE-OS 2852248, the increase in the rate of dissolution of MgO as the additive used in a magnesitically supplied converter, in particular at the beginning of the fresh process while observing the MgO saturation during the entire fresh process and avoiding the addition of flux, has been achieved. The MgO or the substances containing MgO are blown into the focal spot area in the presence of CaO or CaO-containing basic substances at the beginning of the fresh season. Exactly certain amounts of aggregates in certain grain sizes are used for this. Then the required amount of remaining lime is added. This process leads to a rapid dissolution of the additives due to the high temperature in the focal spot area. However, the high MgO contents required to protect the converter lining are only achieved after a reaction time, albeit a short one. After about 20% of the fresh time, there is a slag saturated with MgO. In addition, a blowing lance is required to blow in the additives, which is suitable for blowing in solids. The initial slag is not yet saturated with MgO using this process.

The basicity of the initial slag is also as low as is known from the classic LD process.

Furthermore, the increase in the basicity of the initial slag and thus the reduction in the MgO and CaO contents leading to saturation can be achieved by using substances with a high basicity and low melting point, e.g. Converter slag can be reached before or at the start of freshness.

The use of converter slag is known and has been published, for example, in patent specification FR-PS 1 509 342. This describes a process for the refining of pig iron using liquid converter slag. It is characteristic of this process that the necessary slag additives (lime and silicon oxide surcharge) are introduced in granular form in order to avoid ejection when filling the pig iron and that the converter must be rotatable about its longitudinal axis in a horizontal position. This is a special variant of the LDAC process, in which the final slag always remains in the converter and is slagged after about 50% of the blowing time.

Working with liquid converter slag is also known from “Steel in the USSR” from August 1972, pp. 608-611 (Kuznetsov and others). In order to accelerate slag formation and increase basicity, 20 to 25% of the amount of slag from the previous melt is retained. The slag is thickened with lime and the entire scrap is placed. Then the pig iron is inserted. The slag is made inactive. Working with larger quantities of slag is not technically mastered due to ejection.

Working with retained slag is also described in “Metallurg” 9,1975, pages 18-20 (Kuznetsov et al.). This is braced by lime and scrap before the pig iron is filled. But here too, only blowing with a maximum of 50% of the slag remaining in the converter can be technically controlled.

It is also known to reuse solid, processed converter slag. ("Revue de Metallurgie", May 1978, pp. 297-301, author R. Ando and "Technical Reports Metallurgical Processing Metallurgical Practice", October 1978, pp.789-796, author H. Nashiwa and others). The preferred amount of solid slag is 25 kg / t of steel and is used after 3 blow minutes with a chute. In the middle phase of the fresh process, dolomite is used in an amount of 30 kg / t steel to avoid ejection (with 18% Mg0 in the dolomite). With such a procedure, the slag cannot be saturated over the entire fresh period; only the final slags reach the MgO saturation. However, the main wear of the converter is caused by the slag of low basicity and a high solubility in MgO, especially at the start of blowing. In Japan, this procedure is carried out to reduce the industrial waste (here LD slag).

The object of the present invention is to greatly increase the basicity of the initial slag and to keep the slag as possible over the entire fresh process, but at least in the critical initial region at the saturation limit. Since the invention is intended to apply to all basic brickwork, that is to say to magnesitic and dolomitic infeed, in addition to the MgO saturation, the slag must also be saturated with CaO or with dicalcium silicate. In the known and cited prior art, compliance with the double saturation of 2 Ca0-Si0 ₂ and at the same time of MgO has not been achieved over the entire fresh process and also not during the critical initial phase.

• - dass die Endschlacke der vorhergehenden Charge nach Blasende und Stahlabstich im Konverter verbleibt,
• - dass dieser Schlacke vor bzw. bei Blasbeginn mit den Zuschlägen zur Schlackenbildung eine Menge von 5,0 bis 9,5 kg MgO je t Stahl in Abhängigkeit vom Si-Gehalt des Roheisens nach dem Nomogramm zur Ermittlung des MgO-Zusatzes (b) (Bild 2) zugesetzt werden,
• - dass anschliessend des Roheisen eingefüllt wird,
• - dass danach der Schrott eingesetzt wird,
• - dass dann soviel CaO zugesetzt wird, dass die Sättigung der Schlacke an CaO, bzw. Dikalziumsilikat erreicht wird,
• - dass bei Blasende in Abhängigkeit vom Si-Gehalt im Roheisen von 0,4 bis 1,0% mit einer Schlackenmenge von etwa 120 bis etwa 390 kg je t Stahl nach dem Schlackenmengendiagramm (a) (Bild 1) gearbeitet wird,
• - dass nach Blasende die Hälfte der Schlacke abgezogen wird und die im Konverter verbleibende Restmenge bei der folgenden Schmelze Verwendung findet.

It was surprisingly found that the. Double saturation of the slag when using Mg0 at the start of blowing can only be achieved if, compared to the known, classic LD process, a high proportion of liquid slag remaining in the converter is used. To achieve the object, a method according to the preamble is proposed in claim 1, which is characterized by the following inventive method combination:

• - that the final slag of the previous batch remains in the converter after the end of the blow and steel tapping,
• - That this slag before or at the start of blowing with the additions for slag formation an amount of 5.0 to 9.5 kg MgO per t steel depending on the Si content of the pig iron according to the nomogram for determining the MgO addition (b) ( Image 2) can be added
• - that the pig iron is then filled in,
• - that the scrap is then used,
• - so much CaO is then added that the slag is saturated with CaO or dicalcium silicate,
• - that at the end of the blow, depending on the Si content in the pig iron from 0.4 to 1.0%, the amount of slag from about 120 to about 390 kg per t of steel is worked according to the slag quantity diagram (a) (Figure 1),
• - that half of the slag is drawn off after the end of the blowing and the remaining amount remaining in the converter is used in the subsequent melt.

With this procedure, only the initial and final slags of MgO are saturated. The saturation at 2 CaO. Si02 is achieved over the entire fresh period.

• - dass die Endschlacke der vorhergehenden Charge nach Blasende und Stahlabstich im Konverter verbleibt,
• - dass dieser Schlacke vor bzw. bei Blasbeginn mit den Zuschlägen zur Schlackenbindung eine Menge von 5,0 bis 9,5 kg MgO je t Stahl in Abhängigkeit vom Si-Gehalt des Roheisens nach dem Nomogramm zur Ermittlung des MgO-Zusatzes (b) (Bild 2) zugesetzt werden,
• - dass anschliessend das Roheisen eingefüllt wird,
• - dass so viel CaO zugesetzt wird, dass die Sättigung der Schlacke an CaO, bzw. Dikalziumsilikat dann erreicht wird,
• - dass nach etwa 25 bis 30% der Blasezeit der Schrott eingesetzt wird,
• - dass bei Blasende in Abhängigkeit vom Si-Gehalt im Roheisen von 0,4 bis 1,0% mit einer Schlackenmenge von etwa 120 bis etwa 390 kg je t Stahl nach dem Schlackenmengendiagramm (a) (Bild 1) gearbeitet wird,
• - dass nach Blasende die Hälfte der Schlacke abgezogen wird und die im Konverter verbleibende Restmenge bei der folgenden Schmelze Verwendung findet.

Furthermore, a method according to the preamble is proposed according to claim 2, which is characterized by the following inventive method combination:

• - that the final slag of the previous batch remains in the converter after the end of the blow and steel tapping,
• - That this slag before or at the start of blowing with the additions for slag binding an amount of 5.0 to 9.5 kg MgO per t steel depending on the Si content of the pig iron according to the nomogram for determining the MgO addition (b) ( Image 2) can be added
• - that the pig iron is then filled in,
• - that so much CaO is added that the slag is saturated with CaO or dicalcium silicate,
• - that the scrap is used after about 25 to 30% of the blowing time,
• - that at the end of the blow, depending on the Si content in the pig iron from 0.4 to 1.0%, the amount of slag from about 120 to about 390 kg per t of steel is worked according to the slag quantity diagram (a) (Figure 1),
• - that half of the slag is drawn off after the end of the blowing and the remaining amount remaining in the converter is used in the subsequent melt.

With this procedure, the slag is saturated with 2 CaO · SiO ₂ during the entire fresh process. The MgO levels approach the saturation limit. This procedure is characterized by better solubility of the slag for the added additives because the temperature rises steeply during the fresh process in the absence of the scrap (Fig. 7).

The temperature fluctuates around 1525 ° C when the scrap is used, it can oscillate between 1500 and 1550 ° C and increases to around 1625 ° C at the end of the fresh process. The higher temperature range at the start of blowing promotes the dissolution of the dolomite and lime used.

An advantageous continuation of claim 2 results in claim 3. Here, when the amount of CaO minus the amount of CaO in the dolomite is used, only about 20 to 50% of the necessary amount of CaO is used. The remaining amount is only used together with the scrap after about 25 to 30% of the blowing time. Since the cooling effect of the lime is also reduced at the beginning of the blowing process, an even higher temperature is achieved than in the method according to claim 2 and thus a further improved solubility of the slag compared to the solid additives. With this procedure, the slag is heterogeneous throughout the entire fresh process and contains 2 CaO. Si02 and saturated with MgO.

Surprisingly, it was found with this driving style that the FeO content of the slag at the start of blowing is very low when sampling after 30% of the blowing time. In the classic LD process, the FeO contents are well over 20% and, in combination with the acidic slags, lead to a strong fire-resistant attack. In the described procedure according to the invention, the FeO contents can be reduced to 5%.

Surprisingly, it was found that the amount of CaO required in the classic LD process can be reduced by the amount of MgO required for the MgO saturation of the slag (according to nomogram b in Figure 2). The measure derived from this for carrying out the method is set out in claim 4.

According to claim 5, it is also advantageous if the amount of MgO required to saturate the slag is blown in together with a fresh agent in fine-grained form from the beginning of the blowing up to about 25 to 30% of the blowing time. Furthermore, the amount of MgO required for the saturation of the slag can be introduced in the form of dolomite, the amount of CaO introduced via the dolomite being taken into account when adding CaO.

In all process variants, according to the invention, a quantity of slag is used at the beginning of the blow, as can be seen from the slag quantity diagram a in Figure 1 and which corresponds to approximately twice the amount at the end of the blow compared to the known LD process without using slag (Figure 1). In addition, according to the invention, the amount of MgO added is set according to a nomogram to determine the amount of MgO added (Figure 2) as a function of the silicon content of the pig iron at the start of blowing. In addition, according to the invention, so much CaO is added that the slag is saturated with CaO or dicalcium silicate.

It has been shown that the combination according to the invention of the slag from the previous melt remaining in the converter and the use of the additives dolomite, lime and scrap before or at the beginning of the fresh phase resulted in a completely new, unexpected slag flow. The decisive factor was the finding that the melts could be blown without ejection. The advantages of this procedure can be considered in several ways. On the one hand, the durability of the converter lining is significantly improved by setting the double saturation of 2 CaO · SiO ₂ and MgO during the entire blowing time or at least during the critical initial phase. On the other hand, the amount of additives to saturate the slag with MgO could be considerably reduced by the retention of the liquid converter slag. The slag guidance is substantially evened out. The double saturated slag of 2 CaO · SiO ₂ and MgO is heterogeneous and forms a protective coating on the converter lining. MgO is preferably saturated with mervinite, monticellite and magnesiowustite; in the initial slags primarily via mervinite and monticellite and in the final slags only via magnesiowustite. The delayed use of scrap compared to the classic LD process continues to favor the dissolving conditions of the dolomite or lime used due to the higher initial temperatures of the process. In addition, it was found that the sulfur distribution between metal and slag was significantly improved and that much better final sulfur contents could be achieved. The dephosphorization is also improved compared to the normal LD process.

• % CaO während des Frischprozesses. %Si⁰ ₂

Figure 3 gives an overview of the influence of the residual slag remaining in the converter on the basicity of the slag (expressed by the ratio

• % CaO during the fresh process. % Si ⁰ ₂

Representing the normal LD process is the course of the basicity according to Ijmuiden, according to the designation DE-AS 2 427 205 “Process for the preparation of metal based on iron”, Applicant: Murton, Crawford, B., Pittsburgh, Pa. (USA) and according to the results for normal initial slags at Stahlwerke Peine-Salzgitter AG (P + S).

In the second, middle curve are the Contain basicity values, which have become known from the literature through the reuse of partial quantities of slag, here around 5 t with a 200 t converter (25 kg slag per t steel). An influence on the basicity of the initial slag can already be seen here.

From the third, upper curve, it can be seen that the basicity of the slag is further increased over the entire melting process if the normal amount of slag remains in the converter at the end of the blowing and is used during blowing with approximately twice the amount of slag according to the slag quantity diagram a (Figure 1) becomes. As a result of this measure, the basicity of the slags, which are usually very acidic in the first third to half of the bubble time, are considerably increased and the converter wear caused by the acidic slags is avoided.

The slag composition during the blowing process is described in more detail by Figure 4, by the representation of the course of the slag composition in the three-substance system (CaO + MnO + MgO) '- FeO'-SiO ₂ . While the slag passes through the unsaturated area of FeO- and Si0 ₂ -rich slag at the start of blowing in the normal LD process, this area is no longer touched when working with higher basicity and simultaneous use of Mg0 and the slag is reached or remains in the entire melting time Area of 2 CaO.Si02 saturation (-5% MnO and -10% MgO). When blowing with approximately twice the amount of slag according to the invention, the slags move from the final slag towards the 2 CaO. Si0 ₂ connection and return to the final slag. In the process according to the invention, substantially lower FeO contents are set in the slag over the entire blowing process. The change in the slag composition during the blowing process is therefore much less than in the normal LD process and takes place in the area of more basic, more iron-oxidized slags, which result in significantly less converter wear.

The saturation of the slag with MgO is described in Fig. 5. While in the normal LD process the acidic slags have to dissolve 15 to 20% MgO to achieve the MgO saturation, in the process according to the invention, when working with high basicity, the slag remaining in the converter to only set an MgO content of 8 to 10% in the starting slag. Dolomite is most suitably used as the magnesite carrier. The time of use is before or at the start of blowing.

An overview of the basicity, the MgO contents and the contents of dissolved MgO (MgOg) during the blowing time can be found in Figure 6. The MgO contents with different procedures differ, they start from the same contents and end at the same contents. While the MgO saturation of the slag is achieved over the entire melting time by the method according to claim 3, only the initial and final slags of MgO are saturated in a procedure according to claims 1 and 2.

The process sequence according to the invention will be explained in the following using three examples, which are representative of numerous operational tests, for the claimed process variations.

Example 1 is intended to illustrate the process according to claim 1.

First, the final slag has to be formed. For this purpose, an LD melt is produced in a 200 t converter using the usual method. The dolomite is also set before the start of blowing. The required amount of MgO can be found in the nomogram in Figure 2.

At the end of the blowing, the entire liquid slag (22t) remains in the converter. 49.7 tons of scrap are used for the following melt. Based on the pig iron analysis, the lime scale is 11.9t. The CaO content of the dolomite is subtracted from this according to the following calculation.

In accordance with the nonogram for determining the MgO addition (Figure 2), the following pig iron analysis uses: C = 4.65%, Si = 0.72%, Mn = 0.55%, P = 0.10% and S = 0.011% a quantity of MgO of 7.90 kg per t of steel, ie 1580 kg required for a 200 t melt. With a MgO content of 37%, 4270 kg of dolomite must be used.

With a share of 58% CaO, the dolomite brings in a CaO amount of 2470 kg. Since the lime contains 92% CaO, this corresponds to a quantity of 2680 kg lime. The lime scale must therefore be reduced by this amount. 11.9-2.68 = 9.22t lime are used after the filling of 172.7t pig iron. Then 4.27 t of dolomite are added. The fresh process then begins. As a control, the process for taking a steel and slag sample was interrupted after 4 minutes = 30% of the blowing time. At the end of the fresh process after 13.7 minutes, the final temperature was 1614 ° C. The steel and slag analyzes and the basicity after 30% and 100% of the bubble time showed the following values:

After tapping the steel, 50% of the slag (22t) is drawn off. The removed slag is measured volumetrically in the slag bucket or weighed in the slag bucket. In the further melting sequence, 50% of the slag is always drawn off, so that twice the amount of slag according to the slag quantity diagram a (Figure 1), based on the classic LD process, remains in the converter. The initial and final slag is saturated up to 15% bubble time and from 85% bubble time in MgO, as can be seen in Figure 6.

• Aufgrund des Siliziumgehalts des Roheisens beträgt die MgO-Menge (nach Bild 2) 7,2 kg/t Stahl; das entspricht bei einer 200-t-Schmeize 1400 kg MgO. Bei einem Gehalt von 37% MgO im Dolomit beträgt die einzusetzende Dolomitmenge 3890 kg. Der CaO-Anteil im Dolomit beträgt 58%, das sind 2256 kg CaO und bei 92% CaO im Kalk ergibt das eine Kalkmenge von 2450 kg. Aufgrund der Roheisenanalyse beträgt der Kalksatz 10,16 t, hiervon sind 2,45 t abzuziehen, so dass die einzusetzende Kalkmenge 7,71 t beträgt.

Example 2 describes the operating conditions according to claim 2. The formation of the final slag takes place as already described in claim 1. There are 22 tons of residual slag in the converter. 172 t of pig iron are filled with the following pig iron analysis: C = 4.59%, Si = 0.6%, Mn = 0.52%, P = 0.10%, S = 0.010%. Then the lime and then the dolomite are set. The respective quantities are calculated as follows:

• Due to the silicon content of the pig iron, the amount of MgO (according to Figure 2) is 7.2 kg / t steel; for a 200 t melt, this corresponds to 1400 kg MgO. With a content of 37% MgO in the dolomite, the amount of dolomite to be used is 3890 kg. The CaO content in the dolomite is 58%, that is 2256 kg CaO and with 92% CaO in the lime this results in a lime quantity of 2450 kg. Based on the pig iron analysis, the lime scale is 10.16 t, from which 2.45 t have to be deducted so that the amount of lime to be used is 7.71 t.

After a blowing time of 3.92 minutes (27.3% blowing time) the converter is flipped over and a steel and slag sample is taken. After using 50 tons of scrap, the melt is blown to completion. The end of the blow was 14.35 minutes. The steel temperature was 1622 ° C. A steel and Schlakke sample is taken. After tapping 203 t of steel, half of the slag is emptied, so that 22 t of slag remain in the converter for the subsequent melt. The steel and slag analyzes and the basicity of the slag after approx. 30% and after 100% blowing time gave the following values:

As can be seen from Figure 6, the slag is saturated with MgO in this mode of operation up to 20% and from 80% blowing time.

Example 3 illustrates the process according to claim 3.

The formation of the final slag takes place here first as described in Example 1. The converter then contains 22 t of slag. For this purpose, 175.2 tons of pig iron are filled with the following pig iron analysis: C = 4.62%, Si = 0.68%, Mn = 0.51%, P = 0.10%, S = 0.009%. Then part of the lime is placed. The amount of lime and dolomite is calculated according to the following scheme.

The necessary amount of MgO can be taken from the nomogram (Figure 2) with 7.5 kg / t steel, which corresponds to 1500 kg MgO for a 200 t melt. With a share of 37% MgO in the dolomite, the amount of dolomite is 4050 kg. The CaO portion of the dolomite is (at 58%) 2350 kg, corresponding to 2550 kg of lime. The lime scale is 10.45t based on the pig iron analysis; this leaves 2.55 t so that a quantity of 7.9 t lime remains; of this amount, 2.5 tons of lime are used. The calculated quantity of 4.05 t of dolomite is then set. Then the fresh process begins. After 30% of the bubble time (3.92 minutes of blowing) the process interrupted, a steel and slag sample is taken and a quantity of 47.2 t scrap and the remaining lime quantity of 5.4 t are set. After 13.67 minutes the melt was completely blown and the final temperature was 1639 ° C. The steel and slag analyzes and the basicity after 30% and after 100% of the bubble time showed the following values here:

As can be seen in Figure 6, the slag is saturated with MgO from the beginning of the blowing to the end of the blowing.

Claims (6)

1. Method for the production of steel in a basic converter using fluid converter slag, whereby a final slag is obtained which is saturated with MgO and CaO or dicalcium silicate, characterised in that:

- the final slag of the previous charge remains in the converter after termination of the blowing process and after steel tapping;

- 5.0 kg to 9.5 kg MgO per ton of steel, in dependence upon the Si content in the pig iron, are added to this slag prior to, or at, the commencement of the blowing process together with the additions for slag formation, in accordance with the graph for determining the MgO addition (b) (fig. 2);

- the pig iron is subsequently introduced;

- the scrap metal is then introduced;

- so much CaO is then added that the slag becomes saturated with CaO or dicalcium silicate;

- at the end of the blowing process, in dependence upon the Si content in the pig iron of from 0.4% to 1.0% the operation is carried out with a slag quantity of from approximately 120 kg to approximately 390 kg per ton of steel, in accordance with diagram (a) illustrating the slag quantity (fig. 1);

- after the end of the blowing process, half of the slag is removed, and the residual quantity remaining in the converter is used in the subsequent smelt.

2. Method for the production of steel in a basic converter using fluid converter slag, whereby a final slag is obtained which is saturated with MgO and CaO or dicalcium silicate, characterised in that:

- the final slag of the previous charge remains in the converter after termination of the blowing process and after steel tapping;

- 5.0kg to 9.5kg MgO per ton of steel in dependence upon the Si content in the pig iron, are added to this slag prior to, or at, the commencement of the blowing process together with the additions for slag formation, in accordance with the graph for determining the MgO addition (b) (fig. 2);

- the pig iron is subsequently introduced;

- so much CaO is then added that the slag becomes saturated with CaO or dicalcium silicate;

- the scrap metal is introduced after approximately 25% to 30% of the blowing time;

- at the end of the blowing process, in dependence upon the Si content in the pig iron of from 0.4% to 1.0%, the operation is carried out with a slag quantity of from approximately 120 kg to approximately 390 kg per ton of steel, in accordance with diagram (2) illustrating the slag quantity (fig. 1);

- after the end of the blowing process, half of the slag is removed, and the residual quantity remaining in the converter is used in the subsequent smelt.

3. Method according to claim 2, characterised in that, for the introduction of the CaO only approximately 20% to 50% of the required CaO quantity is introduced and the remaining quantity of CaO required is introduced, together with the scrap metal, after approximately 25% to 30% of the blowing time.

4. Method according to claims 1, and 3, characterised in that the required CaO quantity is reduced by an amount corresponding to the MgO quantity set according to the graph (b).

5. Method according to claims 1, and 3, characterised in that the MgO quantity required to saturate the slag is blown-in in fine-grained form, together with an oxidising agent, starting from the commencement of the blowing process and continuing up to approximately 25% to 30% of the blowing time.

6. Method according to claims 1, and 3, characterised in that the MgO quantity required to saturate the slag is introduced in the form of dolomite, whereby the CaO quantity introduced over the dolomite has to be taken into account for the CaO addition.

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