JPH06256836A - Production of high cleanliness and ultra-low carbon steel - Google Patents

Abstract

PURPOSE:To produce a high cleanliness and ultra-low carbon steel by charging reducing agent on slag in a ladle to execute the slag reforming after tapping the produced undeoxidized molten steel into a ladle and executing vacuum degassing treatment. CONSTITUTION:Molten iron. from a blast furnace is charged into a steelmaking furnace (e.g. converter) to execute the decarburization, and after making C concn. about 0.03-0.06%, this molten steel is tapped into the ladle and introduced into a vacuum vessel in the vacuum degassing apparatus. Oxidizing gas is supplied and the vacuum decarburizing treatment is executed to adjust the molten steel to the prescribed ultra-low carbon concn. At the time of completing the vacuum decarburizing treatment, the suction-up of the molten steel into the vacuum vessel is stopped. The reducing agent of Al, Al ash, Si, etc., is added to the slag in the ladle to make T.Fe concn. in the slag <= about 5%. By this method, the cleanliness of the ultra-low carbon steel can drastically be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultra-low carbon steel, and more particularly to producing ultra-low carbon steel having high cleanliness.

[0002]

2. Description of the Related Art The melting of ultra-low carbon steel is carried out by tapping the undeoxidized molten steel having a C concentration of about 0.03 to 0.06 wt% (hereinafter simply referred to as "%") melted in a steelmaking furnace into a ladle. Then, a reducing agent was added to the slag on the molten steel, and the T.O. It is general to perform so-called slag reforming to reduce the Fe concentration, and subsequently perform vacuum decarburization treatment with a vacuum degassing device.

As a method of modifying this slag, a reducing agent such as Al, Al ash and Si is put into the slag on the molten steel in the ladle, and further, Japanese Patent Publication No. 2-19168 and Japanese Patent Laid-Open No. 30711/1990. As disclosed in each publication, it is more effective to stir the slag after adding the reducing agent.

[0004]

However, when the ladle slag is reformed according to the conventional melting method and subsequently the vacuum degassing apparatus performs the vacuum decarburizing treatment by supplying the oxidizing gas, the slag is Reoxidation of the T. It was found that the Fe concentration increased. Therefore, the effect of the slag modification before the vacuum decarburization treatment, that is, the T.
By reducing the Fe concentration, oxygen in the slag reacts with Al and the like in the steel to prevent the oxygen concentration in the steel from increasing, and the effect of increasing the cleanliness of the steel decreases. Then, since the cleanliness of the molten steel is lowered, it is difficult to reduce the occurrence of the nozzle clogging during casting and the surface defects of the product due to the decrease in the cleanliness.

An object of the present invention is to solve the above problems and propose a method capable of mass-producing ultra-low carbon steel with high cleanliness.

[0006]

According to the present invention, undeoxidized molten steel melted in a steelmaking furnace is tapped into a ladle, and the molten steel introduced into the vacuum tank of a vacuum degassing apparatus is oxidized. In the melting method of ultra-low carbon steel that supplies gas and performs vacuum decarburization treatment,
After the vacuum decarburization treatment is completed, a reducing agent is put on the slag in the ladle to modify the slag in the ladle, which is a method for melting a highly clean ultra-low carbon steel.

The term "after vacuum decarburization treatment" is not limited to "immediately after decarburization treatment". For example, when degassing or deoxidation treatment (killing treatment) is performed after decarburization treatment, slag is removed after these treatments. It also includes performing reforming.

[0008]

Operation: Fig. 1 shows that after adding 1 kg of deoxidizer per 1 ton of molten steel to ladle slag after tapping in the ladle, the slag is reformed by stirring the slag by bottom blowing of Ar and then RH. When the vacuum decarburizing treatment was performed while supplying oxygen to the molten steel bath surface introduced into the vacuum tank of the vacuum degassing apparatus, the T.V. in the slag before and after the vacuum decarburizing treatment was performed. This shows the Fe concentration.

[0009] From the figure, T. Fe ≦ 8%
When the slag modification of No. 1 was performed, the T.O. The Fe concentration is the same as that of the T. It is understood that the Fe concentration becomes higher than the Fe concentration, that is, the effect of the slag reforming is drastically reduced.
In the vacuum decarburizing treatment, oxygen is supplied to the molten steel to increase the oxygen concentration, causing reoxidation of the slag, and T. This is because the Fe concentration increases. And T. If the Fe concentration is high, the effect of the above-mentioned slag modification cannot be expected, so that it becomes difficult to increase the cleanliness of steel.

Therefore, in order to carry out the slag reforming more reliably, adding a reducing agent onto the slag in the ladle after the vacuum decarburizing treatment can be a very effective means. Furthermore, the slag modification can be performed with higher efficiency by stirring the slag with a bottom-blown or top-blown lance, a stirring rod, or the like after adding the reducing agent.

Next, the method of the present invention will be described in detail. First, the hot metal from the blast furnace is charged into a steelmaking furnace, for example, a converter, where decarburization is mainly performed. The blowout stop C concentration in this converter is preferably 0.06% or less. This is because if it exceeds 0.06%, the oxygen concentration in the steel decreases, resulting in poor decarburization, which may lead to problems such as a longer processing time. On the other hand, the lower limit of the C concentration is preferably 0.03%. The reason for this is that if the blowing stop C concentration is lower than 0.03% ,. T. in slag As the Fe concentration increases, the effect of slag reforming decreases. The oxygen concentration in steel exceeds the concentration required for decarburization, and the excess deoxidizer basic unit increases. This is because problems such as severe wear of the converter refractories occur.

Next, after the molten steel is tapped in a ladle, D
A H or RH vacuum degasser is used to achieve a predetermined carbon concentration. That is, according to the carbon concentration and the dissolved oxygen concentration obtained in the steps up to the above, and further the molten steel temperature, etc., it is arranged in the vacuum tank of the vacuum degassing device, for example, from the upper blowing lance, the steel bath surface in the vacuum tank. The surface is sprayed with oxygen or an oxidizing gas containing oxygen. Here, when the dissolved oxygen concentration is insufficient, the sprayed oxygen serves as an oxygen source in the steel and contributes to an increase in the decarburization rate, and some oxygen burns the CO gas generated by decarburization. Becomes CO ₂ and the heat of combustion is transferred to the molten steel.

Then, adjusting to a predetermined extremely low carbon concentration,
After completing the vacuum decarburization treatment, stop the sucking of molten steel into the vacuum tank and modify the slag in the ladle, or, following the vacuum decarburization treatment, reduce Al etc. in the vacuum tank. Deoxidizing molten steel by adding a chemical agent (killing treatment), and if necessary, further adjusting components, etc., after this treatment, sucking of molten steel is stopped and slag in the ladle is modified. In the modification of the slag in the ladle, a reducing agent such as Al, Al ash and Si is added to the slag, and the T. Fe concentration 5%
The following is preferable.

[0014]

【Example】

(Example 1) C: 0.05% undeoxidized molten steel was melted in a converter, tapped in a ladle, and then vacuumed on the molten steel bath surface introduced into the vacuum tank of the RH vacuum degassing apparatus. Oxygen gas is blown at a flow rate of 5 to 50 Nm ³ / min from a water-cooled lance vertically inserted from the top to the bottom of the tank to perform vacuum decarburization treatment to decarburize to C: 0.0025%, and 1t of molten steel is placed in the vacuum tank. The RH vacuum degassing process was terminated by introducing 1.0 kg of Al per liter and performing the kill treatment, and then 1.5 kg of Al ash per 1 ton of molten steel was added to the slag in the ladle, and bubbling of top-blown Ar was performed 3 times. The slag was sufficiently stirred for 1 minute to reform the slag.

(Example 2) C: 0.05% undeoxidized molten steel was melted in a converter, tapped in a ladle, and then on a molten steel bath surface introduced into a vacuum tank of an RH vacuum degassing apparatus. In addition, 5 to 50 Nm ³ / mi of oxygen gas was introduced from the water cooling lance vertically inserted from the top to the bottom of the vacuum chamber.
Vacuum decarburization is performed by spraying at a flow rate of n and C: 0.0025
% Decarburization, the RH vacuum degassing process was interrupted, 1.5 kg of Al ash was added per 1 ton of molten steel on the slag in the ladle, and bubbling of top-blown Ar was performed at a flow rate of 5 to 50 Nm ³ / min.
After the slag was sufficiently agitated by performing the heating for 1 minute to reform the slag, the RH vacuum degassing treatment was restarted, and 1.0 kg of Al per 1 ton of molten steel was charged into the vacuum tank to perform the kill treatment.

(Comparative Example) C: 0.04% undeoxidized molten steel was smelted in a converter, tapped in a ladle, and then molten steel 1 was placed on the slag in the ladle.
While adding 1.5 kg of Al ash per t, bubbling of top-blown Ar at a flow rate of 5 to 50 Nm ³ / min for 3 minutes to sufficiently stir the slag, and then introduce it into the vacuum chamber of the RH vacuum degasser. Oxygen gas was supplied onto the molten steel bath surface at a flow rate of 5 to 50 Nm ³ / min for vacuum decarburization treatment, and then 1.0 kg of Al per 1 ton of molten steel was charged into the vacuum tank for killing treatment.

The ultra low carbon steels thus obtained were supplied to a tundish and subjected to continuous casting. In Figure 2,
In the above-mentioned Examples 1 and 2 and Comparative Example, the T.V. in the slag immediately after tapping from the converter to the tundish. The change of Fe concentration is shown. From the figure, in Examples 1 and 2 according to the present invention, the T.D. The Fe concentration was lower than that of the comparative example according to the conventional method, and it can be seen that the slag was effectively modified.

Further, as shown in FIG. 3, the number of surface defects in the products obtained through continuous casting, hot rolling and then cold rolling under the same conditions is shown as an index. The surface defects in the cold rolled material made of the ultra low carbon steels obtained in Nos. 1 and 2 were reduced by half compared to the cold rolled material according to the comparative example,
It can be seen that it has greatly contributed to quality improvement. The surface defect index is an index of the number of defects and the length per 10 m of the coil.

[0019]

According to the present invention, the cleanliness of ultra-low carbon steel can be remarkably enhanced, and therefore the improvement of product quality can be advantageously achieved.

[Brief description of drawings]

FIG. 1 T.O.S. in slag before and after RH vacuum degassing treatment. It is a graph which shows Fe concentration.

FIG. 2 T. slag. 5 is a graph comparing Fe concentrations.

FIG. 3 is a graph comparing surface defects of products after cold rolling.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Saburo Moriwaki, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Inside the Chiba Works, Kawasaki Steel Co., Ltd. (72) Masaaki Kuga 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Chiba Steel Works, Ltd.

Claims (1)

[Claims] 1. Vacuum decarburization by tapping undeoxidized molten steel melted in a steelmaking furnace into a ladle and supplying an oxidizing gas to the molten steel introduced into the vacuum tank of a vacuum degassing device from this ladle. In the method of melting ultra-low carbon steel to be treated, after the vacuum decarburization treatment, a reducing agent is put on the slag in the ladle, and the slag in the ladle is reformed. Method for melting low carbon steel.