JPS6191313A - Method for refining molten steel - Google Patents

Abstract

PURPOSE:To manufacture easily low P steel by discharging molten steel from a converter into a ladle refining furnace at a relatively low temp. in an undeoxidized state, removing molten slag, and deoxidizing the molten steel. CONSTITUTION:Since the P content in molten steel increases in proportion to the rise in the temp. of the molten steel, molten steel is discharged from a converter into a ladle refining furnace at a relatively low temp. such as 1,600-1,650 deg.C so that the P content in the molten steel is reduced when the molten steel is discharged. The molten steel is discharged in an undeoxidized state without adding a deoxidizer or a ferroalloy. Molten slag contg. P is removed to prevent the rephosphorization of the molten steel in the refining furnace, and a deoxidizer, a ferroalloy and other composition regulating agent are added. Low P steel of desired quality is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for melting low-strength square steel using a ladle refining furnace.

(Prior art) Generally, when tapping molten steel, the tapping temperature is raised to around 1,700°C to compensate for the temperature, and a deoxidizing agent and ferroalloy are added to perform deoxidized tapping. . In the ladle, the composition is adjusted by bubbling in the presence of slag.

(Problem to be solved by the invention) However, as shown in the relationship between the tapping temperature and the final point CP), as the tapping temperature increases, the content of the final point CP) tends to increase. However, when steel is tapped at high temperatures as in the past, the phosphorus content in the molten steel increases. Moreover, the conventional
Since this is already deoxidized steel and the slag is still present in the ladle, the phosphorus in the slag is re-phosphoroused into the molten steel when bubbling, etc. is performed in the ladle. There is a problem with this. For this reason, with the conventional method, it has been possible to reduce the Fi and phosphorus contents in molten steel to about tsoppm at most.

The present invention has been made in view of the above circumstances, and its object is to provide a method that can refine molten steel with a lower phosphorus content than conventional methods.

(Structure of the Invention) The present invention taps undeoxidized molten steel from a converter at a tapping temperature of 1,600 to 1,650°C to lower the phosphorus content at the time of tapping, and also uses a deoxidizing agent, ferroalloy, etc. at the time of tapping. In the ladle refining furnace, slag is sufficiently removed to prevent rephosphorization, and in this state, ferroalloy and deoxidizer are added to adjust the composition and deoxidize the steel. A method is used in which a ladle smelting furnace is heated to compensate for the temperature.This makes it possible to obtain molten steel with a significantly lower phosphorus content than in the conventional method.

The present invention will be described in detail below.◎ In the present invention, the tapping temperature from the converter is set at 1600 to 1650.
℃. This tapping temperature is 1700℃ compared to the conventional tapping temperature.
As a result, the phosphorus content contained in the molten steel during tapping can be reduced (see drawing).

Furthermore, in the present invention, the steel is extracted without being deoxidized without adding a deoxidizing agent or alloy □iron. If a deoxidizing agent or the like is added, there is a risk that this will react with the slag and the phosphorus in the slag will return to the molten steel.
In the present invention, since a deoxidizing agent and the like are not added during tapping, this problem can be solved. Note that a dephosphorizing agent such as sodium metasilicate or a mixture of lime and iron oxide may be added during tapping.

Next, in the present invention, slag is sufficiently removed from the ladle refining furnace containing the tapped molten steel. As mentioned above, if slag is present, the phosphorus in the slag transfers to molten steel due to bubbling etc. and rephosphorus is generated, but in the present invention, since the slag is sufficiently removed, rephosphorus does not occur. The slag is preferably removed by a conventional method, for example by vacuum slag suction treatment (VSC).

With the slag sufficiently removed in this way, a deoxidizer and a ferroalloy are added and stirred. The types and amounts of these additives are the same as in the conventional method. Even if these additives are added, there is no risk of rephosphorization since there is no slag. Temperature compensation is performed by increasing the temperature during addition of a deoxidizing agent, etc., and stirring. The temperature to be raised is, for example, about 50°C to 80°C, and the temperature of the molten steel is 163°C.
The temperature should be around 0℃.

Then, the molten steel refined in this way is continuously cast, for example, through an RH treatment process or without an RH treatment process.

(Effects of the invention) According to the method of this invention, the tapping temperature is lowered and deoxidizing agent and ferroalloy are added in a state without slag, so rephosphorization is reliably prevented and the phosphorus is lower than that of the conventional method. Refining the content of molten steel. be able to.

For example, in the case of ordinary hot metal, the phosphorus content is 30% in the conventional method.
However, according to the present invention, it can be suppressed to 150 ppm or less. In the case of dephosphorized hot metal, the phosphorus content is about 150 ppm in the conventional method, but according to the present invention, it can be suppressed to 5'Oppm or less.

(Example) LD undeoxidized steel (LD slag 15
yu/ton) and then add 4 liters of sodium metasilicate in a ladle refining furnace.
The mixture was stirred for 15 minutes with argon gas at a flow rate of 0.5 Nl/min.

After removing the slag by vacuum suction treatment, ferroalloy and deoxidizing agent are added and stirred in an argon atmosphere.
The molten steel in the ladle refining furnace was arc heated to 1630° C. and the treatment was completed.

When we investigated the phosphorus content of the raw steel obtained by this method, we found that the phosphorus content was extremely low: 60 to 1,100 ppm for ordinary pig, and 40 to 80 ppm for the mixture of ordinary pig and pre-treated pig iron. Karita: On the other hand, the tapping temperature was 1700.
℃, and at the time of tapping, a deoxidizing agent and ferroalloy were added, and the steel was stirred in a ladle in the presence of slag.The phosphorus content of the raw steel obtained by this method was investigated. However, the concentration was 150 to 200 ppm in ordinary pig iron, and 100 to 150 ppm in the mixture of ordinary pig and pretreated pig iron.

[Brief Description of the Drawings] The drawings are characteristic diagrams showing the relationship between the tapping temperature and the content at the end point CP).

Claims (1)

[Claims] After undeoxidized molten steel is tapped from a converter at a tapping temperature of 1,600 to 1,650℃, slag is removed in a ladle refining furnace, and a deoxidizer and ferroalloy are added and stirred to deoxidize and adjust the composition. A method of refining molten steel in which the temperature is compensated by increasing the temperature.