JP2923182B2 - Melting method of ultra low carbon steel - Google Patents

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 by vacuum degassing of molten steel. The present invention relates to a method for improving the product quality by controlling the dissolved oxygen concentration in the steel. The present invention relates to a vacuum degassing method.

[0002]

2. Description of the Related Art Ultra-low carbon steel is manufactured by vacuum degassing and degassing of undeoxidized or weakly deoxidized molten steel in a steelmaking furnace by RH method, DH method or other ladle refining methods. It is generally well known that charcoal treatment is performed. For example, JP-A-2-213410, JP-A-4-22851
No. 5 publication. The ultra-low carbon material by this refining is manufactured into a continuously fired thin sheet, a non-oriented electrical steel sheet, and the like. In order to improve the quality or electromagnetic properties of those products, it is preferable that the C content of the product steel sheet is as low as possible. .

When performing vacuum degassing / decarburization treatment in the production process of ultra-low carbon steel, it is preferable to increase the concentration of dissolved oxygen in the steel in order to increase the decarburization efficiency. However, after the vacuum degassing / decarburizing treatment, the dissolved oxygen in the molten steel is changed to Al, S
It is deoxidized by i, Ti, etc., thereby producing deoxidation products, many of which remain as inclusions in the state of being suspended in the steel. These inclusions increase as the concentration of dissolved oxygen in the molten steel during deoxidation increases.

[0004] High levels of inclusions in steel have many adverse effects on steelmaking operations and product quality characteristics. In steelmaking operations, nozzle narrowing of molten steel pots, nozzle narrowing of tundishes, and inclusions of immersion nozzles occur, and on the quality characteristics of products, internal defects and surface defects on the product plate may occur. In addition, there are problems such as deterioration of magnetic properties such as magnetic flux density or iron loss of the functional material.

As described above, when ultra-low carbon steel is smelted by vacuum degassing and decarburization, the higher the dissolved oxygen concentration in the steel, the more advantageous it is to improve the decarburization efficiency. The lower the dissolved oxygen concentration in steel is, the more advantageous in improving operability and product quality / characteristics, and both require conflicting operating conditions. Therefore, in order to satisfy both, it is important to control the dissolved oxygen concentration in the steel to a certain appropriate range according to each steel type.

In order to control the dissolved oxygen concentration, it is necessary to increase the carbon concentration at the end of refining in the refining furnace or to control the dissolved oxygen concentration by adding iron oxide, blowing oxygen, etc. Is used in vacuum smelting.

[0007]

However, in the conventional vacuum refining such as the RH method and the DH method in which the dissolved oxygen concentration is controlled, in order to control the dissolved oxygen concentration by the blow-off carbon concentration, the blow-off carbon concentration is required. Variations occur, and the dissolved oxygen concentration itself differs greatly even at the same carbon concentration.

[0008] Controlling the dissolved oxygen concentration by adding iron oxide, blowing oxygen, etc., causes a decrease in the temperature of the molten steel due to a decomposition reaction, a variation in the oxygen concentration, a prolonged treatment time, and a loss of refractories. There are drawbacks such as a large capital investment required.

The present invention solves the above-mentioned conventional problems, reduces the inclusions as much as possible, and performs vacuum degassing without adversely affecting the refractory and operability to melt ultra-low carbon steel. It is to provide a way to do it.

[0010]

In order to achieve the above object, undeoxidized molten steel or weakly deoxidized molten steel melted in a steelmaking furnace is reduced to an extremely low level by an RH method, a DH method, or another vacuum degassing apparatus. In a vacuum degassing method for melting carbon steel (extremely low carbon steel having a product C content of 100 ppm by weight or less), the dissolved oxygen concentration in the steel after vacuum degassing is 350 ppm.
Then, a very low carbon steel is melted by adding a deoxidizing material.

In the above-mentioned vacuum degassing method,
There is a method for producing ultra-low carbon steel in which a carbon material is added at the time of vacuum degassing to reduce the dissolved oxygen concentration in the steel after the treatment to 350 ppm or less, and then a deoxidizing material is added.

Further, in order to control the dissolved oxygen concentration in the steel to a required level, a small amount of carbon material is continuously added during the vacuum degassing process, and the continuous charging of the carbon material is performed by vacuum degassing and decarburizing processes. It is preferable to control the dissolved oxygen concentration to a predetermined value by performing the method in the first half.

[0013] The continuous addition of a small amount of carbonaceous material is carried out by feeding it into a vacuum tank through a supply device from a hopper installed above the degassing equipment, or by putting a small amount of it in practice into a large number of times. Is also good.

In addition, it is necessary to carry out carbon material charging in the first half of the vacuum degassing and decarburizing treatments.
1 or half of the vacuum decarburization process time until deoxidation with Si, Mn, Ti, etc., or the carbon material is charged within 10 minutes, whichever is shorter, to improve the vacuum decarburization process efficiency. Can be almost eliminated.

As described above, by controlling the dissolved oxygen concentration in the steel after the treatment (before the addition of the alloy) to 350 ppm or less in the vacuum decarburization treatment, it is possible to prevent nozzle clogging during continuous casting, stabilize the operation, and improve the quality. In addition, the product characteristics can be improved.

Further, FIG. 1 shows a case in which pitch coke is added as a carbon material in an amount of 20 to 40 kg and 40 to 60 kg during the vacuum decarburization process by charging a carbon material in the first half of the vacuum degassing and decarburizing process. Despite high dissolved oxygen concentration in steel before vacuum decarburization treatment, dissolved oxygen concentration in steel before alloy addition is 350 pp
m, and the absolute value itself can be controlled lower. This means that the addition of the carbon material is more effective in preventing nozzle throttling, stabilizing operation, and improving quality and product characteristics.

FIG. 2 shows the decarburization rate constant in the case where the above-mentioned carbonaceous material is charged to perform a decarburization treatment with a low dissolved oxygen concentration in steel. By adding the material, the decrease in the decarburization rate constant is slightly suppressed, and the decrease in the dissolved oxygen concentration in the steel is remarkable.

[0018] This is because the carbon material added in order to put the carbon material in the steel in the first half of the vacuum degassing and decarburizing treatment to a sufficiently high level is directly oxidized considerably in the tank, so the decarburization rate is high. The decrease in the constant is slight, and conversely, the effect of decreasing the concentration of dissolved oxygen in the steel increases. The refining of the present invention is based on Al, Si,
In the case of deoxidation with Mn or the like, inclusions increase in the steel and the steel is contaminated, but the addition of the carbon material is a CO or CO ₂ reaction, and the oxygen concentration can be controlled without contamination of the steel by the product.

[0019]

EXAMPLES The results of vacuum degassing and decarburization treatments using the present method by the DH method with a processing capacity of 350 t are shown in Table 1 and FIG.
As shown in FIG. Nos. 1 to 4 are vacuum degassing and decarburizing treatments according to the present invention. 5 is a comparative example.

[0020] There was a concern that the decarburization rate could be reduced by adding carbonaceous material during the degassing process to control oxygen in the steel. In the first half, or within 10 minutes after the start of the degassing treatment, whichever is shorter, the reduction in the decarburization rate was extremely small. For this reason, the oxygen in the molten steel at the initial stage of degassing is at a sufficiently high level, and the carbon material added in a small amount is directly oxidized and removed in the tank. Since the exhaust capacity of the degassing device in the initial stage of degassing is sufficiently high, it is considered that there is almost no effect on the exhaust performance of the generated gas due to the oxidation of the carbon material added in small amounts.

Also, as shown in FIG. 3 and FIG. 3 and FIG. 4, the present method controls the dissolved oxygen concentration in the steel before adding the alloy to 350 ppm or less to change the opening degree of the casting nozzle. (Nozzle throttling) is surely prevented, and the surface flaws of the product are significantly improved as shown in Table 1 as well as in FIG. 4, indicating that this method is excellent.

[0022]

[Table 1]

[0023]

As described above, by using the vacuum degassing and decarburizing treatment of the present invention to reduce the dissolved oxygen concentration in the steel before alloy addition to 350 ppm or less, the opening degree of the casting nozzle changes (nozzle throttle). ) Is reliably prevented, and the occurrence of product surface defects can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the concentration of dissolved oxygen in steel at the start of decarburization treatment and the concentration of dissolved oxygen in steel before alloy addition after the treatment.

FIG. 2 is a diagram showing the relationship between the concentration of dissolved oxygen in steel and the decarburization rate constant before the addition of an alloy, and shows that the decarburization rate constant is almost the same when the carbon material is added in FIG.

FIG. 3 is a graph showing a dissolved oxygen concentration in steel and a nozzle opening degree change rate before adding an alloy.

FIG. 4 is a graph showing the relationship between the concentration of dissolved oxygen in steel before the addition of an alloy and the incidence of product surface defects.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-19417 (JP, A) JP-A-63-190113 (JP, A) JP-B-49-23732 (JP, B1) (58) Field (Int.Cl. ⁶ , DB name) C21C 7/10 C21C 7/06 C21C 7/068 C21C 7/00 101

Claims (2)

(57) [Claims] In a vacuum degassing method for melting undeoxidized molten steel or weakly deoxidized molten steel into ultra-low carbon steel by vacuum degassing and refining in a steelmaking furnace, the steel after vacuum degassing is used. A method for melting ultra-low carbon steel, comprising reducing the concentration of dissolved oxygen to 350 ppm or less and then adding a deoxidizer. 2. A vacuum degassing method in which undeoxidized or weakly deoxidized molten steel melted in a steelmaking furnace is melted into ultra-low carbon steel by vacuum degassing and refining. To reduce the concentration of dissolved oxygen in the steel after the treatment to 350 ppm or less, and then to add a deoxidizing agent.