JP3253138B2 - Melting method of high cleanness 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-clean carbon steel having high cleanliness.

[0002]

2. Description of the Related Art Conventionally, in order to produce ultra low carbon steel, undeoxidized molten steel having a low carbon concentration produced in a converter is decarburized using an RH reflux degassing apparatus. A deoxidizing treatment is performed by adding a deoxidizing alloy such as Al.

[0003]

However, the RH
The slag in the ladle in the recirculation type degassing device contains a high concentration of lower oxides such as iron oxide because the slag flowing out of the converter is mixed therein. When the deoxidizing treatment is performed, the deoxidizing element dissolved in the molten steel is oxidized by the oxygen from the slag to generate nonmetallic inclusions, and there is a problem that the cleanliness of the molten steel cannot be improved.

[0004] As a method of improving the cleanliness of molten steel,
For example, as disclosed in Japanese Patent Application Laid-Open No. 2-30711, after Al or Al slag is added to ladle slag to reduce iron oxide in the ladle slag, decarburization treatment is performed using an RH reflux degassing apparatus. And subsequently deoxidizing Al.
In order to smelt ultra-low carbon steel having a total oxygen concentration [TO] of 20 ppm or less in molten steel by such a method, the content of [T.Fe] in the slag after the deoxidization treatment is at least 2% or less, desirably. Although it is necessary to reduce it to 1% or less, considering the variation in the amount of slag flowing out of the converter and the increase in iron oxide concentration in the slag during the decarburization treatment, [T.Fe] in the slag after the treatment with RH is considered. In order to stably reduce the temperature, the molten steel 1
An operation was required in which 0.5 to 1.0 kg of metallic Al per t was added to the ladle to make the excess of Al.

[0005] However, if such an excessive amount of Al is present during the decarburization treatment, Al reacts with the dissolved oxygen in the molten steel, so that the oxygen concentration of the molten steel is reduced and the decarburization rate is significantly reduced. there were. A method of performing the slag reduction treatment after the decarburization treatment is also conceivable. However, in this method, the presence of carbon pickup from the reduction flux and the RH
After the treatment with slag, the reactivity of the slag has decreased,
Since a stirrer for accelerating slag reduction is required, and it is necessary to perform deoxidation treatment again using RH after slag reduction treatment, the treatment time by RH per batch becomes longer, and RH treated steel is reduced. There were problems such as a significant decrease in productivity.

[0006] It is an object of the present invention to provide a method for melting ultra-clean carbon steel having high cleanliness, which solves the above-mentioned problems.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a method for producing ultra-low carbon steel in which undeoxidized molten steel is decarburized by an RH reflux degassing apparatus and then deoxidized. Before the treatment by the device, the metal Al-containing substance is added to the ladle slag to reduce lower oxides such as FeO and MnO in the ladle slag, and then MgO is mainly used immediately after the start of reflux in the RH reflux degassing device. After adding the flux as a component into the RH degassing tank, decarburizing treatment is continued until the target carbon concentration becomes lower than the target, and deoxidizing treatment is further performed by adding a deoxidizing alloy. This is a method of melting ultra-low carbon steel.

[0008]

According to the present invention, when a ladle slag is subjected to a reduction treatment with a metal Al-containing substance and then subjected to a decarburization treatment with RH, a flux containing MgO as a main component immediately after the start of RH reflux is removed from the RH degassing tank. To form an MgO-enriched layer between the undeoxidized molten steel and the unreacted metal Al-enriched layer, thereby suppressing the unreacted metal Al from being entrained in the molten steel, thereby reducing the metal Al. Unreacted metal Al
Even when a large amount of is present, it is possible to prevent a reduction in the decarburization speed at the initial stage of decarburization.

[0009] In addition, when there is no unreacted metal Al, in the conventional method, the concentration of iron oxide in the slag increases due to the transfer of oxygen from the molten steel during the decarburization treatment. However, in the present invention, the increase in the iron oxide concentration in the slag is largely suppressed by the effect of the addition of MgO, and the iron oxide concentration in the slag during the deoxidation treatment performed after the decarburization treatment can be reduced, so that the cleaner is more clean. A very low carbon steel with a high degree can be melted.

As described above, in the present invention, even when the degree of reduction of the ladle slag and the degree of residual unreacted metal Al are different due to, for example, variations in the iron oxide concentration in the converter steel slag and the amount of slag flowing out of the converter. Since the concentration of iron oxide in the slag of the deoxidizing treatment can be reduced without inhibiting the decarburization reaction, extremely low carbon steel with higher cleanliness can be efficiently and stably melted.

The amount of metal Al added when the metal Al is charged into the ladle is preferably in the range of 0.3 to 1 kg / t per ton of molten steel. 0.3kg
If it is less than / t, the reduction of iron oxide in the slag may be insufficient. Mg added at the start of RH treatment
The addition amount of O-based flux is MgO per ton of molten steel.
A range of 1 to 4 kg / t is good. If MgO is less than 1 kg / t, the effect of blocking unreacted metal Al and molten steel is not sufficient, and if the amount of metal Al added to the ladle slag exceeds 1 kg / t, decarburization inhibition in the early stage of decarburization is observed. Also, if the amount of MgO added to the ladle slag exceeds 4 kg / t, the deoxidation rate during the deoxidation treatment tends to decrease, and the absorption rate of alumina inclusions into the slag decreases. Conceivable.

[0012]

EXAMPLES The results of implementing the method of the present invention at a RH of 260 tons of molten steel will be described below. 0.04% carbon concentration in converter
Immediately after tapping the undeoxidized molten steel to the ladle,
After adding 50% Al pure Al slag to the pot slag, 300Nl / min of Ar gas was blown from a porous tuyere provided at the bottom of the pot and stirred for 3 minutes to reduce the slag. Thereafter, vacuum decarburization treatment was performed using RH as shown in FIG. 1, but when the reflux started about 30 seconds after the start of evacuation, the MgO pure fraction 95
% Or more MgO clinker was added. At the beginning of the decarburization process,
In the vacuum chamber, oxygen gas was blown upward at a blowing speed of 30 Nm ³ / min for about 3 minutes. After decarburization treatment until the target carbon concentration became 25 ppm or less, metal Al was added to adjust the Al concentration in the molten steel to a range of 0.04 to 0.05%, and deoxidation treatment was performed for 10 minutes after the addition of Al.

The amount of MgO added at the start of the RH treatment is 2 kg /
FIG. 2 shows the relationship between the basic unit of the metal Al for reducing the ladle slag and the total oxygen concentration [TO] in the molten steel after the deoxidizing treatment in the case of t. When the basic unit of metal Al is 0.4 kg / t or more, the total oxygen concentration [TO] in molten steel can be stably reduced to 30 ppm or less. Even if the basic unit of metal Al is 1 kg / t or more,
It can be seen that the effect on cleaning of molten steel is small only by an increase in unreacted components.

FIG. 3 shows the relationship between the basic unit of the metal Al for reduction treatment of ladle slag and the decarburization treatment time, and the effect of MgO addition on this. By adding 2 kg / t of MgO,
Even when a large amount of Al is added, the adverse effect on the decarburization rate is smaller than that of the conventional method in which MgO is not added. The basic unit of metal Al for ladle slag reduction treatment is 1.0 kg / t
FIG. 4 shows the relationship between the MgO addition amount at the start of the RH treatment and the decarburization treatment time in the case of (1). In the method of the present invention in which MgO is added, the decarburization time is greatly reduced as compared with the conventional method in which MgO is not added.If the MgO unit unit is set to 1.0 kg / t or more, decarburization inhibition by the addition of Al slag is prevented. Can be prevented.

Metal unit consumption for ladle slag reduction is 1.0 kg /
In the case of t, the amount of MgO added at the start of the treatment with RH
FIG. 5 shows the relationship of [TO]. Compared with the conventional method in which MgO is not added, in the method of the present invention in which MgO is added, the [TO] tends to decrease slightly, and it is considered that the reoxidation of slag during the decarburization treatment is suppressed. However, when the added amount of MgO was further increased to 4 kg / t or more, [TO] tended to increase, probably because the absorption rate of alumina-based inclusions into slag decreased.

[0016]

As described above, according to the present invention, it is possible to smelt high cleanness ultra low carbon steel with improved cleanliness of molten steel without causing decarburization inhibition during decarburization processing of ultra low carbon steel. Becomes

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an apparatus for carrying out the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a metal Al basic unit for ladle slag reduction treatment and [TO] after deoxidation treatment.

FIG. 3 is a characteristic diagram showing a relationship between a metal Al basic unit for ladle slag reduction treatment and a decarburization treatment time.

FIG. 4 is a characteristic diagram showing the relationship between the amount of MgO added at the start of RH treatment and the decarburization treatment time.

FIG. 5 is a diagram showing the relationship between the amount of MgO added at the start of the RH treatment and [TO].

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-301814 (JP, A) JP-A-2-277711 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21C 7/10 C21C 7/068

Claims (1)

(57) [Claims] In a method for producing ultra-low carbon steel in which undeoxidized molten steel is decarburized by an RH recirculating degasser and then deoxidized, a ladle slag is prepared before the treatment by the RH recirculating degasser. Add metal Al-containing material to FeO,
After reducing the lower oxides such as MnO, the flux containing MgO as the main component was added to the RH degassing tank immediately after the start of reflux in the RH recirculating degassing apparatus, and then decarburized until the target carbon concentration became lower than the target. A method for producing ultra-low carbon steel with high cleanliness, characterized by performing a treatment and further adding a deoxidizing alloy to perform a deoxidizing treatment.