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Cause analysis of bubbles on continuous casting billet surface and improvement measures

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Air bubble is one of the main surface defects of billet in steelmaking.  In 2013, several continuous casting machines of Hangang had bubble defects to varying degrees. The billet with bubble defects was prone to crack and other waste products in the rolling process, which seriously affected the continuous stability of production and also affected the market image of the enterprise.  Based on the production practice of other units and its own technological conditions, this paper analyzes the forms and causes of continuous casting billet bubbles, and takes corresponding improvement measures to achieve the purpose of improving the bubble defects.

Keyword：

Casting billet; Air bubbles

Realed product：Copper mould tube

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• Bubble definition and generation mechanism 

2.1 Bubble Definition 

Below the surface of the billet, the holes growing along the columnar crystal direction are called bubbles.  Relatively small bubbles close to the surface of the billet and densely distributed are called pores.  According to the bubble position, the surface of the surface is called the surface bubble, do not leak out of the surface is called the subcutaneous bubble.

2.2 Bubble formation mechanism 

Casting billet subcutaneous bubble is the so-called subcutaneous slag, mainly due to the quality of the protection slag is not stable, easy to crust, in the case of mold liquid level fluctuation is involved in the steel.  Such bubbles are generally sporadic.

The bubbles on the surface of the billet are the bubbles formed by the gas in the steel. Such bubbles are generally enriched at the solidification interface during the solidification of molten steel. When the total pressure of CO, H2 and N2 is greater than the sum of the static pressure and atmospheric pressure of molten steel, bubbles will be formed.

The picture

• Gas sources in molten steel 

In actual production, gas mainly enters molten steel through two ways: one is molten steel absorbing external gas;  Second, the external water vapor into the molten steel heat decomposition.

3.1 External Gas 

External gas refers to air, protective gas in gaseous form into the gas in the steel.  A large amount of air will be inhaled during steel alloying of converter, LF power supply, large turn of molten steel for wire feeding, open casting of continuous casting or gap of protective pouring device.

On the one hand, oxygen molecules and some nitrogen molecules in the air will dissolve into the steel at the gas-liquid interface, increasing the content of [O] and [N] in the steel.  Some CO 2 in air will react with C, Si, Mn and Al in steel to form metal oxides and carbon monoxide gas.  On the other hand, the undissolved air enters the liquid steel in the form of pores, and some of the nitrogen, oxygen and hydrogen atoms dissolved in the liquid steel will diffuse to the interface in the form of atoms when they contact the boundary of pores existing in the steel, forming nitrogen, oxygen and hydrogen molecules into the pores.

At present, the whole process of casting is protected by argon blowing to prevent secondary oxidation of molten steel in contact with air.  In the process of molten steel pouring from the ladle to the tundish, argon gas enters into the molten steel from the gap between the lower nozzle of the ladle and the long immersed nozzle of the ladle, and then escapes from the surface of the molten steel in the ladle, and bubbles will not enter the crystallizer basically.  Between the ladle and crystallizer, some argon gas enters into the molten steel at the stopper rod in the ladle and the gap between the upper and lower water outlets in the ladle, and moves to different parts of the crystallizer at a certain depth with the molten steel.  The temperature of liquid steel in the corner of mold is relatively low, so it is difficult for bubbles to float up, and they are easily captured by solidified billet shell, and finally form subcutaneous bubbles.

3.2 External water vapor 

External water vapor sources mainly refer to: converter, refining equipment leakage;  Alloy, slag, protective slag and covering agent and other materials moisture exceeds the standard;  The capacity of the secondary cooling fan of the continuous casting machine is insufficient, and the steam comes out from the mold cover plate and enters the mold after cooling.  Poor baking of refractory materials such as tundish.

3.2.1 Device Water Leakage

The water leaked from the equipment is heated into water vapor, which increases the partial pressure of water vapor.  The higher the partial pressure of steam in furnace gas, the higher the hydrogen content in molten steel.  After molten steel is cast and cooled, the solubility of gas decreases and a large amount of gas is precipitated, resulting in subcutaneous bubble defects in billet.

3.2.2 Raw material moisture exceeds the standard

As part of the direct contact with molten steel raw materials containing large amounts of water under high temperature decomposition, the product dissolve Yu Gangshui form [H] and [O], [O] in the formation of oxide inclusions in steel, and had a high solubility in the high temperature liquid steel [H] as cooling solidification of liquid steel in the steel solubility decreases gradually, part of [H] precipitation, precipitation form small bubbles of hydrogen,  The bubbles not escaping from the surface of the billet form subcutaneous bubbles, while the small bubbles exposed on the surface of the billet form pinhole defects.

3.2.3 Secondary cooled steam

Continuous casting process, casting machine water-cooling system to produce water vapor, caused by a lack of ability of exhauster, steam will rise along the casting machine, in the mold plate under surface water droplets, condensed from the mold copper plate edge into the top of the mold, into the mould protection slag, and even part of the water vapor from the combined mould Angle into the slot and rising, into powder, lead to wet slag,  And slagging in meniscus, resulting in continuous casting.  Some of this water vapor can enter the sintering layer, decompose into [H], [0] atoms, and finally enter the molten steel through the slag layer.

3.2.4 Water in refractories

Plug rod and other refractory baking effect is not good, casting the first few billets, even the first whole furnace will occur bubble defects.  This is because the resistant material and high temperature molten steel contact, hidden in the resistant material steam volatilized into the steel into H], [O] atoms, part of the formation of CO and H 2 bubbles, and will generate part of the oxide inclusion.  As casting goes on, the billet bubbles will gradually decrease.  In addition, the bubble defect in the billet head may be caused by the secondary oxidation of molten steel due to the poor effect of pouring protection.

• Experimental comparison 

4.1 Adjust the flow rate of argon for continuous casting protection 

Aiming at the bubble defect of casting billet, the argon gas flow rate of plug rod and upper nozzle was adjusted first, and the experiment of increasing, decreasing and completely closing was carried out on the basis of the original argon gas flow rate.  The results show that when the flow rate of argon increases, the bubbles in the billet increase.  The bubbles are slightly reduced but not completely eliminated.  There was no obvious change in bubbles after complete closure.

4.2 Sampling the water content of continuous casting protective slag and covering agent 

The moisture content of protective slag and covering agent of continuous casting from different suppliers has been tested for many times. The results show that the moisture content of most varieties is above 0.5%, and the moisture content of some varieties is above 1.0%.

Through the comparison of the tests, it can be concluded that the water content of protective slag and covering agent exceeds the standard is the main reason for the bubble defect of billet.  The large flow control of protective argon also promotes the formation of bubbles in the billet.

• measures to eliminate billet bubbles 

(1) Strengthen the moisture spot check of protective slag, covering agent and other raw materials;  Protective slag is baked in the oven before use;

(2) On the premise that the nozzle is not blocked and the liquid slag layer of the crystallizer is active, the injection amount of argon gas is reduced;

(3) It is advisable to insert the depth of the nozzle into the liquid steel level, and it is strictly prohibited to splash;

(4) Before each pouring, argon is blown in the tundish for more than 2 minutes to drive the air;

(5) Improve the baking system of refractory materials such as medium package;

(6) It is forbidden to bilge the large volume of molten steel and breathe in contact with the air;

• the conclusion 

There are many reasons for casting billet bubble defects, which can be divided into two categories: external gas and external water vapor.  By strictly controlling the water content of raw materials such as protective slag and covering agent, and adjusting the flow rate of argon gas, the bubble defects of casting billet have been effectively solved.[/vc_column_text][vc_column_text]One stop solution for steel industry[/vc_column_text][/vc_column][/vc_row]