JPH08246026A - Method for controlling form of inclusion in molten steel - Google Patents

Abstract

PURPOSE: To make it possible to efficiently and finely disperse a large amt. of nonmetallic inclusions in molten steel by supplying a specific quantity of oxygen component to the molten steel after adding a deoxidizing agent thereto. CONSTITUTION: A chamber 4 in which an argon atmosphere is maintained is internally provided with a vacuum high-frequency furnace 1 where the molten steel 2 is melted. The molten steel 2 is subjected to component adjustment and, thereafter, molten steel is cast into a die casting mold 8 via a tundish 7 three to four minutes after addition of the deoxidizing agent. The oxygen component of 1×10<-2> to 6×10<-2> kg per 1 ton of the molten steel in a killed state is supplied to the molten steel. The supply of the oxygen component is executed by blowing gas of an oxygen content of 1 to 10vol.% into the molten steel. The gas is blown via a porous plug 5 from a gas supply source 6. As a result, the form of the nonmetallic inclusions existing in the molten steel is controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling inclusion morphology of molten steel for controlling the morphology of non-metallic inclusions present in molten steel.

[0002]

2. Description of the Related Art Non-metallic inclusions mainly composed of oxides are present in molten steel, and when the amount thereof is excessive, not only nozzle clogging occurs in a short time, but also non-metallic inclusions are removed. If it remains in the steel even after solidification, it causes a serious defect in the steel product. For this reason, conventionally, non-metallic inclusions such as deoxidation products are thoroughly removed in each step of the steelmaking process so that the amount of non-metallic inclusions in molten steel does not increase.

However, as an idea contrary to the purpose of the clean steel to reduce such non-metallic inclusions, the steel material characteristics are improved by positively increasing the inclusions having a grain size small enough not to cause product defects. Attempts have been made to improve it. For example, “Materials and Processes” 3 (19
90), p.276, and "Iron and Steel", 78 (1992), 1697, propose a method of controlling the dispersion of sulfide by using oxides in steel as precipitation nuclei of sulfide.

As a method for finely dispersing a large amount of core oxides, the composition of the produced oxide is changed by adjusting the molten steel component and the slag component, and an oxide having a large specific gravity such as Zr deoxidation is produced from the molten steel. There has been proposed a method of reducing the levitation ratio and increasing the residual ratio in molten steel.

[0005]

However, even if these methods are used, the number of inclusions dispersed in the steel is not sufficient and the distribution thereof is large, and
If Zr remains in the steel, it has a great influence on the material hardness, so that there are problems such as the addition amount and the addition method being difficult.

The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a method for controlling the morphology of inclusions in molten steel capable of efficiently finely distributing a large amount of non-metallic inclusions in the molten steel. To aim.

[0007]

The method for controlling the inclusion morphology of molten steel according to the present invention is 1 × 10 ^{-2 to} 6 × 10 ^-2 per ton of molten steel in a killed state after adding a deoxidizing agent to the molten steel. kg
It is characterized by supplying the oxygen content of.

In this case, it is desirable to blow a gas having an oxygen content of 1 to 10% by volume into the molten steel in the killed state. In addition, as the primary deoxidizing agent, Si, Mn, T
It is preferable to use a metal or compound containing a component element such as i.

[0009]

In the method of controlling the inclusion morphology of molten steel according to the present invention, the dissolved oxygen content defined by the deoxidization equilibrium of each element or compound is added after adding a deoxidizing agent such as Si, Mn, and Ti to the molten steel. Except the above, a so-called primary deoxidation product is produced corresponding to the initial oxygen content. After that, since the amount of oxygen that can be dissolved decreases as the molten steel cools and solidifies, a secondary deoxidation product is produced corresponding to the excess oxygen content.

Non-metallic inclusions generated by the flow and stirring in the molten steel are removed to the outside of the system, but some of the primary deoxidation products and most of the secondary deoxidation products remain in the molten steel. However, it is brought into the steel ingot even after being cooled and solidified.

Usually, the secondary deoxidation product has a smaller particle size than the primary deoxidation product. The reason for this is that in the secondary, the amount of dissolved oxygen gradually decreases as the temperature drops and the amount of the deoxidation product is small, so that a large amount of the deoxidation product is not generated instantaneously as in the primary. Furthermore, in the secondary, there is no time to allow the inclusions to aggregate and coalesce to enlarge.

Therefore, in order to distribute a large amount of fine inclusions having a small particle size, it is conceivable to increase the amount of secondary inclusions defined by the deoxidation equilibrium value of the component elements of the deoxidizer. Therefore, after adding the deoxidizing agent, oxygen is forcibly supplied into the molten steel to bring it into a non-equilibrium state, thereby restarting the production of the deoxidized product that had once been suspended, and increasing fine inclusions. However, if the oxygen supply amount becomes excessive, the conditions for the primary generation are close to those at the molten steel interface, so that coarse inclusions with a large grain size are likely to occur. That is, in order to distribute a large amount of fine inclusions, appropriate oxygen supply conditions exist.

The reason for setting the lower limit of the oxygen content supply amount to 1 × 10 ^-2 kg per ton of molten steel in the killed state is that the number of fine inclusions of 3 μm or less sharply increases from around this as shown in FIG. Because it will increase. On the other hand, the reason for setting the upper limit of the oxygen content supply amount to 6 × 10 ^-2 kg per ton of molten steel in the killed state is that the oxygen supply amount becomes excessive and coarse inclusions with a large grain size are likely to occur. is there.

[0014]

Embodiments of the present invention will be described below with reference to the accompanying drawings. (Embodiment 1) A vacuum high frequency induction furnace 1 is provided in a chamber 4 in an argon gas atmosphere as shown in FIG. 1, and molten steel 2 is melted in the furnace. After adjusting the composition of the molten steel 2 melted in the furnace, a deoxidizer was added 3 to 4
After a minute, it was cast into a mold mold 8 through the tundish 7. The molten steel 2 after the composition adjustment is C; 0.08 to 0.12% by weight, Si; 0.02 to 0.06% by weight, Mn; 0.8.
.About.1.0% by weight, P; 0.007% by weight or less, S;
005% by weight or less, Ti: 0.02 to 0.15% by weight,
Al: The composition is 0.001% by weight or less. The weight of the molten steel 2 whose composition has been adjusted is about 5000 kg.

The tundish 7 is preheated before receiving the molten steel 2. Next, the addition of oxygen will be described. Add the amount of iron oxide (Fe ₂ O ₃ ) added to the reagent to 0.0
Various additions were made to the molten steel 2 within the range of 8 to 1.0 kg.
10 minutes per minute through the porous plug 5 embedded in the bottom of the furnace
Argon gas was blown into the molten steel 2 at a flow rate of 0 Nl and sufficiently stirred. After a lapse of 3 to 4 minutes from the end of the addition of iron oxide, this was cast in the mold mold 8 and solidified.

The cooling rate of the steel ingot was measured with a thermocouple at a position of about D / 4 (1/4 of the mold diameter D) in the radial direction of the mold. The measurement result of the cooling rate was about 20 ° C. per minute.
A total of 9 samples were cut out from three locations in the radial direction D / 2, D / 4, and D / 8 at the three locations of the upper portion, the central portion, and the lower portion of the steel ingot. The cut surface of each sample was mirror-polished and the polished surface was observed under a microscope with a magnification of about 800 times to measure the amount and distribution of inclusions per cm ^{2 of the} surface to be inspected.

The number of non-metallic inclusions was counted by dividing the particle size by 1 μm. The total number of inclusions in the sample was set as a representative value by averaging the measured values of the above 9 samples.

In FIG. 2, the horizontal axis represents the oxygen intensity per ton of molten steel (× 10 ^-2 kg / ton), and the vertical axis represents the number of inclusions having a diameter of 3 μm or less per 1 cm ² of steel cross section. It is a characteristic diagram which shows the result of having taken (pieces /) and investigated about the influence which the oxygen basic unit (oxygen addition amount) has on the amount of inclusions. As is clear from the figure, the oxygen intensity is 1 x 1
The number of fine inclusions having a diameter of 3 μm or less increased at 0 ⁻² kg / ton or more. From these results, although not at the level of this time, it is expected that the oxygen concentration at the interface will increase, the inclusions will tend to enlarge, and the number of fine particles will decrease if the oxygen unit amount is further increased. (Example 2) Using the same apparatus as in Example 1 above, a method of supplying oxygen as a gas was performed. Both the argon gas and the oxygen gas were blown into the molten steel 2 from the gas supply source 6 through the porous plug 5 at the bottom of the furnace and sufficiently stirred.

In this case, the blowing gas amount is 500 per minute.
-1500 Nl, and the oxygen basic unit is fixed at 0.01 kg per ton of molten steel, and the oxygen concentration in the gas is 0-
Various changes were made within the range of 10% by volume.

After a lapse of 3 to 4 minutes from the end of the gas blowing, this was cast in the die mold 8 and solidified. The same evaluation method as in Example 1 was used to examine the non-metallic inclusions contained in the ingot.

In FIG. 3, the horizontal axis represents the oxygen concentration (volume%) in the blown gas, and the vertical axis represents the number of non-metallic inclusions having a diameter of 3 μm or less (pieces / cm ² ) per 1 cm ² of the steel material cross-sectional area.
FIG. 4 is a characteristic diagram in which the influence of the oxygen concentration in the blown gas on the number of inclusions is investigated by taking the above. Curve G in the figure shows the result when the blowing gas amount is 500 Nl / min,
Curve H shows the result when the amount of blown gas is 1500 Nl / min.

As is clear from the figure, the number of inclusions increased when the oxygen concentration in the blown gas was 1 to 10 (volume%). Especially, the oxygen concentration in the blown gas is 3 to 8% by volume.
In the range, the number of fine non-metallic inclusions having a diameter of 3 μm or less was remarkably increased.

[0023]

EFFECTS OF THE INVENTION According to the present invention, a non-metallic inclusion in the molten steel can be efficiently added in a large amount by supplying an oxygen content more appropriately into the molten steel that has been once made into a killed state by adding a deoxidizer. It can be finely dispersed.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an outline of an apparatus used for a method for controlling inclusion morphology of molten steel according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing the effect of the present invention.

FIG. 3 is a characteristic diagram showing the effect of the present invention.

Claims (2)

[Claims] 1. An inclusion form of molten steel, characterized by supplying 1 × 10 ^{−2 to} 6 × 10 ⁻² kg of oxygen content per ton of molten steel in a killed state after adding a deoxidizing agent to the molten steel. Control method. 2. A gas having an oxygen content of 1 to 10% by volume is blown into molten steel in a killed state.
A method for controlling the inclusion morphology of the molten steel described.