KR20010028401A - method for feeding Bi-wire into molten steel - Google Patents

Abstract

PURPOSE: A method for adding bismuth to molten steel is provided to increase working safety by restraining precipitation of bismuth so that bismuth is prevented from being flown into a casting nozzle sliding gate and a ladle bubbling gas blowing hole, and decrease defects of surface by using bismuth having a fine particle size. CONSTITUTION: The method for adding bismuth to molten steel (1) comprises the process of bottom bubbling the molten steel (1) so that the molten steel is agitated in a flow rate of 0.40 to 0.70 Nm3/Hr per ton of molten steel in an opposite direction to the feeding direction of bismuth wire (2) from right before the bismuth wire is fed in manufacturing bismuth free cutting steel by feeding a bismuth wire into the molten steel (1) which is received in a ladle (3). The feeding rate of the bismuth wire (2) is controlled so that wire skin is melted at right below a position where a vapor pressure of the bismuth wire is identical to a steel static pressure. The bismuth wire (2) is fed into the molten steel (1) as being placed on a vertical line of a gas blowing hole (4) of the ladle (3) for bubbling.

Description

Method for feeding Bi-wire into molten steel}

The present invention relates to a method for manufacturing bismuth free cutting steel, and more particularly, to a bismuth addition method that can prevent the bismuth particles from settling to the lower portion of the molten steel to flow out of the casting nozzle sliding gate.

The free cutting steel contains elements for improving machinability such as sulfur (S), lead (Pb), and bismuth (Bi). Currently, bismuth free cutting steel having no significant effect on the human body is widely used because Pb is harmful to the human body.

Generally, bismuth is added to the steel by powder injection in the refining process, cored wire feeding, and mold injection in the continuous casting process. The present invention relates to a method for producing bismuth free cutting steel by a wire feeding method. Bismuth wire is a bismuth or bismuth alloy (Bi-Mn) contained in the bark.

The wire feeding method injects bismuth wire into the molten steel during the refining of the molten steel from the converter process.By bismuth is difficult to input during degassing due to the high vapor pressure. It is injected into the molten steel from the upper ladle. When bismuth wire is introduced into the molten steel, lower bubbling is performed to uniformly mix.

However, bismuth has a specific gravity of 9.8 g / cm 3, which is much larger than iron (7.8 g / cm 3), and has little solubility in molten steel, so when added to molten steel, it is settled to the bottom of the ladle after melting. These metals, which have settled to the bottom of the ladle, penetrate into the casting nozzle sliding gate, making the casting difficult, and block the ladle bubbling plug (bubbling gas inlet) to prevent the lower bubbling. In addition, the surface tension is so small that it penetrates between the refractory and enters between the outer shell and the inner lead weakens the strength of the inner lead. In addition, even when manufactured into a cast through a casting operation, the bismuth compound may be segregated and the cast may break during rolling, which may cause a decrease in the yield of rolling.

Korean Unexamined Patent Publication No. 1997-33252 discloses a technique for manufacturing a high quality bismuth free cutting steel using a wire injection method and a mold injection method at the same time. In this technique, a bismuth free cutting steel having a bismuth free-cutting steel inside and a sulfur free-cutting steel inside is proposed by introducing bismuth wires into a mold of a continuous casting machine near the discharge port of the deposition nozzle. However, there is no mention of bismuth sedimentation problem here either.

An object of the present invention is to provide a method for preventing bismuth from settling to the bottom of the ladle and flowing out of the casting nozzle sliding gate when the bismuth wire is introduced into the molten steel in the ladle.

1 is a graph showing the relationship between bismuth particle size and sedimentation rate

2 is a schematic diagram of bismuth input

Bismuth addition method of the present invention for achieving the above object, the lower bubbling so that the molten steel is stirred in a direction opposite to the input direction of the bismuth wire at a flow rate of 0.40 ~ 0.70N ㎥ / Hr immediately before the bismuth wire into the molten steel It is configured to include.

Hereinafter, the present invention will be described in detail.

The melting temperature of steel bar of ordinary bismuth wire is about 1,530 ℃, and the melting temperature of bismuth-manganese compound is about 1,200 ℃, and because bismuth has lower melting temperature than iron bar, molten steel of high temperature about 1,600 ℃ Injecting the wire into the wire leads to the following phenomenon.

The bismuth present in the powder form of the iron is melted first and coalesced, and the coalesced molten bismuth particles become large, thereby accelerating the sedimentation rate. 1 shows the sedimentation rate according to the bismuth particle size. Here, Stokes means a case where no turbulence is formed (less than 0.1 Reynold), and Newton means a case where a turbulent shape is formed (less than 0.1 Reynold). In other words, when the particle radius is small, it does not form turbulence, so it moves according to Stokes law and the sedimentation rate is rapidly increased, and when it reaches a certain speed, it forms turbulence and becomes seized according to Newton's law. Settling speed is faster. The Stokes law and Newton's law are shown below.

Renold Number (Re) = Dνtρ / η

Where: D: diameter of bismuth droplets, R: radius of bismuth droplets, ρs: specific gravity of bismuth, ρ: specific gravity of molten steel, η: viscosity of molten steel, υt: sedimentation velocity of bismuth droplets

As shown in the above equation, other factors except the size of the droplets of bismuth are already determined, so it can be seen that the size of the bismuth droplets should be reduced in order to reduce the sedimentation rate.

Therefore, the present invention pays attention to minimizing the sedimentation rate by breaking the fine particles of bismuth as fine as possible, and concluded that the molten steel must be strongly stirred for this purpose. In the case of 100 ton ladle, the ladle height is about 3,130mm, and when the time required for the start of casting after the input of bismuth wire is about 15 minutes, it is confirmed that the sedimentation can be prevented when the size of the bismuth particle is 50 μm or less ( 1).

The steel stir of molten steel is maintained just before and after the bismuth wire. Steel stirring methods for molten steel include a method of blowing an inert gas such as argon into molten steel, electromagnetic stirring, and the like, and the bubbling method is preferable in the present invention. The stirring force of the molten steel is controlled by the flow rate of the blown gas. In the present invention, a flow rate of 0.4-0.7 Nm 3 / Hr per ton of molten steel is preferable. If the flow rate is less than 0.4Nm3 / Hr, bismuth will flow out, and if it exceeds 0.7Nm3 / Hr, the molten steel will overflow.

In the present invention, the stirring direction as well as the stirring force is an important factor. As shown in FIG. 2, the stirring direction of the molten steel 1 should be opposite to the feeding direction of the bismuth wire 2. If they match, they will help settling. Preferably, when the bismuth wire is introduced on the vertical line of the gas inlet 4 (aka lower bubbling plug) of the ladle 3, the sedimentation of the bismuth can be suppressed as much as possible.

In FIG. 2, reference numeral 5 denotes a bubble.

The bismuth wire is introduced under the above conditions, but the feeding speed may be in the usual manner, but preferably, the vapor pressure of the bismuth wire is directly below the same position as the iron static pressure, that is, the iron static pressure is higher than the vapor pressure of the bismuth. The feed rate is controlled to melt the iron shell. The vapor pressure of bismuth is shown in equation (4) and the iron positive pressure in equation (5).

[Relationship 4]

log (p _Bi ) = A × (1 / T) + B × log (T) + C × T + D

Where p _Bi is the vapor pressure of bismuth, T is the temperature of the molten steel, and A, B, C, and D are constants.

[Relationship 5]

p _steel = rho × g × h

Where p _{steel is} static pressure, rho is molten steel density, g is gravity acceleration, and h is iron static pressure measurement position

Table 1 below shows the vapor pressure of bismuth according to the molten steel temperature when the constant A is -10400, B is -1.26, C is 0, and D is 12.35.

Molten steel temperature (℃) Bismuth Vapor Pressure (p _Bi ) Molten steel temperature (℃) Bismuth Vapor Pressure (p _Bi ) 1520 0.37 1565 0.50 1525 0.38 1570 0.51 1530 0.40 1575 0.53 1535 0.41 1580 0.55 1540 0.42 1585 0.57 1545 0.44 1590 0.58 1550 0.45 1595 0.60 1555 0.47 1600 0.62 1560 0.48 1605 0.64

Table 2 below shows the iron static pressure by position (h) in molten steel (rho-0.0068 kg / cm 3, vapor pressure of bismuth wire -0.6) at 1595 ° C.

Position (h) Positive _steel (p _steel ) Position (h) Positive _steel (p _steel ) 100 2.03465 1700 1.96025 200 1.9675 1800 0.8931 300 1.90035 1900 0.82595 400 1.8332 2000 0.7588 500 1.76605 2100 0.69165 600 1.6989 2200 0.6245 700 1.63175 2300 0.55735 800 1.5646 2400 0.4902 900 1.49745 2500 0.42305 1000 1.4303 2600 0.3559 1100 1.36315 2700 0.28875 1200 1.296 2800 0.2216 1300 1.22885 2900 0.15445 1400 1.1617 3000 0.0873 1500 1.09455 3100 0.02015 1600 1.0274

As can be seen from the above table, when the molten steel has a temperature of 1595 ° C, the vapor pressure of bismuth is 0.6, and the position of the iron static pressure higher than this vapor pressure is 2200 mm. Therefore, the feed rate is adjusted in consideration of the melting time of the iron so that the iron bar of the wire is melted below this position.

Hereinafter, the present invention will be described in detail through examples.

Example 1

Place the wire so that bismuth wire (steel shell melting time 1.03 sec) is injected into the vertical line of the lower bubbling gas inlet as shown in FIG. The free cutting steel was manufactured by injecting a wire at a speed of 1.10 m / sec while changing the flow rate. At this time, the change of operating conditions according to the lower bubbling flow rate was examined.

Bubbling Flow Rate (N㎥ / Hr) 0.20 0.30 0.40 0.50 0.60 0.70 0.80 Bismuth Spill Occur Occur Not Occurred Not Occurred Not Occurred Not Occurred Overflowing

As described above, according to the present invention it is possible to prevent the bismuth outflow to the casting nozzle sliding gate, ladle bubbling gas inlet, etc. by suppressing the sedimentation of the bismuth can increase the work safety, and the bismuth particles are fine It has a useful effect that can lower the defect rate by reducing the surface defects.

Claims (3)

In the method for producing bismuth free-cutting steel by injecting bismuth wire into the molten steel received in the ladle, the molten steel in the opposite direction to the input direction of the bismuth wire at a flow rate of 0.40 ~ 0.70N ㎥ / Hr per molten steel immediately before the bismuth wire is injected A method of adding bismuth to molten steel, characterized in that it comprises a lower bubbling to be stirred. The method of claim 1, wherein the bismuth wire is added with bismuth to the molten steel, characterized in that the feed rate is adjusted so that the steel bar of the wire melts under the same position as the vapor pressure of the bismuth wire. The method of claim 1, wherein the bismuth wire is placed on a vertical line of the gas inlet of the ladle for bubbling to add bismuth to molten steel.