​Smelting Process and Operational Guidelines for Basic Furnace Steel

Basic Furnace Steel Smelting Process and Precautions

Basic furnace steel is a type of steel smelted in a basic steelmaking furnace. Its furnace top, furnace wall and furnace bottom are all built or sintered with basic refractory materials, such as aluminum-magnesium bricks, magnesia bricks and magnesia sand. The characteristics of this type of steel are that its sulfur and phosphorus content is low, while the gas content is relatively high. With the vigorous development of the industrial revolution in the early 19th century, the demand for steel increased sharply, and at the same time, more stringent requirements were put forward for the quality of steel. It is in this context that people continue to explore more efficient steelmaking technology, and the basic furnace steel smelting process came into being.

The innovation process of the basic furnace steel smelting process can be described as magnificent. In the early 19th century, with the vigorous development of the industrial revolution, the demand for steel increased sharply, and at the same time, more stringent requirements were put forward for the quality of steel. It was in this context that Bessemer proposed a revolutionary steelmaking method in 1856: by placing molten pig iron into a converter and blowing in high-pressure air, impurities such as silicon, manganese, carbon, and phosphorus in the pig iron were burned, thereby producing high-quality steel. The pear-shaped movable converter he invented greatly improved the efficiency of steelmaking. It only took 10 minutes to turn 10 to 15 tons of molten iron into steel, which was significantly more efficient than the traditional stirring method. However, the acid converter invented by Bessemer had limitations in removing phosphorus, so it was mainly suitable for countries such as Sweden and Austria, which were rich in low-phosphorus and low-sulfur iron ore resources.

In 1879, British metallurgist Thomas further proposed the alkaline converter steelmaking method. This method uses dolomite high-temperature sintered clinker mixed with tar to make an alkaline refractory brick lining, and by blowing in air and adding quicklime, the entire reaction is carried out in an alkaline high-temperature environment. This improvement allows the oxidized phosphorus to combine with lime, thereby effectively remaining in the slag, avoiding the problem of phosphorus returning to the steel. Thomas' innovative method was quickly adopted by countries such as Germany and France, which are rich in phosphorus iron ore resources, and further promoted the progress of steelmaking technology.

In addition, the basic arc furnace oxidation steelmaking process is also an important part of modern steelmaking technology. The process mainly includes seven stages: raw material preparation, furnace filling, batching and charging, melting period, oxidation period, reduction period and steel tapping. In the raw material preparation stage, there are strict requirements on the quality of scrap steel, including cleanliness, the content of harmful metal elements and the clarity of chemical composition. Meeting these requirements is crucial to ensuring the quality of steelmaking, reducing costs and improving productivity.

1. Raw Material Preparation

Scrap steel is the main raw material for electric arc furnace steelmaking, and its quality directly affects the composition of molten steel, production safety and cost-effectiveness. The specific requirements are as follows:

Cleanliness requirements: Scrap steel needs to have less rust on the surface and no mud, sand and oil. Pollutants will reduce conductivity, prolong melting time, increase the hydrogen content of molten steel, and affect the dephosphorization effect during oxidation period.

Restriction of harmful elements: Lead (easy to deposit at the bottom of the furnace and cause steel leakage), tin/arsenic/copper (causing hot brittleness), zinc (gasification pollution) and other non-ferrous metals are not allowed to be mixed in.

Safety regulations: It is forbidden to mix in sealed containers, flammable, explosive or toxic substances.

Composition clarity: Storage is classified according to chemical composition, and the sulfur and phosphorus content must be strictly controlled.

Size control: The cross-sectional area of scrap steel is ≤300mm×300mm, and the length is ≤350mm to ensure the charging density and melting efficiency.

2. Furnace Filling Operation

After a furnace of steel is smelted, the furnace needs to be filled before loading to repair the furnace bottom and the corroded slag line and other parts to ensure the normal shape of the furnace body, so as to ensure the smooth progress of the smelting process and safe production. 

The key points of furnace filling include: check the furnace lining immediately after steel is discharged, remove the residue first when filling the furnace bottom, and follow the principles of high temperature, fast filling and thin filling; the filling materials need to be mixed evenly in advance and properly baked after filling the furnace.

3. Batching and Charging

Batching is a key link in electric furnace steelmaking, and its rationality directly affects the smelting efficiency and steel quality. The following points should be noted when batching:

First, ensure the accuracy of batching calculation; second, reasonably match the size of the charge to facilitate loading and rapid melting;

Furthermore, select the appropriate charge according to the quality requirements of the molten steel and the smelting method; finally, the batching composition must meet the process specifications.

Before loading, a layer of lime should be spread on the bottom of the furnace to make the molten slag in advance, which is conducive to early dephosphorization, reduce the steel liquid's air absorption and accelerate the temperature rise. When loading, half of the small charge should be placed at the bottom, and the large charge, low-carbon scrap steel and refractory charge should be placed on the top. Small charges should be added between the large charges, and medium-sized charges should be placed above and around the large charges. Finally, the electrode blocks pre-smashed into 50-100mm in size are loaded into the lower layer of the charge, and ensure that the loaded charge is hemispherical.

4. Melting Period Operation

The melting period is the starting stage of the electric arc furnace steelmaking process. Its task is to quickly melt the solid charge into molten steel and perform dephosphorization at the same time. During this period, it is necessary to pay attention to reducing the absorption of gas by the molten steel and the volatilization of the metal.

Arc starting stage. When the power is turned on and the arc is started, the furnace is full of charge, and the distance between the arc and the furnace top is quite close. In order to avoid damage to the furnace top due to excessive input power or excessive voltage, we usually choose an intermediate voltage and control it to about 2/3 of the rated power of the input transformer.

Well penetration stage. At this stage, the arc is completely surrounded by the charge, and the heat is almost completely absorbed by the charge, so it will not cause damage to the furnace lining. At this time, we can use the maximum power for smelting. The well penetration process takes about 20 minutes, which accounts for about a quarter of the entire melting time.

Electrode rising stage. When the electrode "penetrates" to the bottom, a molten pool will form at the bottom of the furnace. With the oxidation of lime and some elements at the bottom of the furnace, a layer of slag will form on the surface of the molten steel. The surrounding charge continues to melt due to radiant heat, and the increase in the amount of molten steel causes the liquid level to rise, and the electrode gradually rises accordingly. At this stage, we still use the maximum power to transmit electrical energy, and this process takes about half of the total melting time.

During the melting process of the charge, we need to add appropriate amounts of lime and ore in batches to make slag according to the phosphorus content in the charge to facilitate dephosphorization. The amount of lime added is usually controlled between 1% and 2% of the weight of the charge. At the same time, in order to adjust the fluidity of the slag, fluorite can be added in appropriate amounts.

During the melting process, "pushing and melting" should be continued. When most of the charge begins to melt, oxygen blowing can be used to accelerate the melting of the charge. When blowing oxygen, it is necessary to pay attention to shallow blowing to increase the temperature. The depth of insertion into the molten steel should be less than 100mm, the angle should be controlled between 30° and 45°, and the oxygen pressure should be maintained between 0.4 and 0.5MPa.

At the end of the melting, we should use a lower voltage for power supply. After the charge is completely melted, the molten steel needs to be fully stirred, and samples are taken at the center of the molten pool for analysis to grasp the content of carbon, phosphorus, sulfur and other elements in the molten steel. These data will serve as an important basis for subsequent oxidation, reduction reactions and control of element content. If the carbon content in the molten steel is insufficient, then before starting the oxidation reaction, carbon addition must be performed.

V. Oxidation Period

During the oxidation period, we add oxidants to oxidize the carbon in the molten steel, thereby causing the molten pool to boil. The main tasks of this stage are decarburization and dephosphorization, while removing gases and inclusions, and further raising the temperature of the molten steel. The operating process of the oxidation period is as follows:

In the previous stage of the oxidation period, due to the relatively low temperature of the molten steel, our main task is slag dephosphorization. The dephosphorization reaction in the furnace involves multiple chemical reaction steps. First, 5FeO reacts with 2Fe3P to generate P2O5, 11Fe and heat; then, P2O5 reacts with CaO to generate CaO·P2O5 and heat. Through these reactions, we can effectively remove phosphorus from the molten steel.

It can be seen from the above reaction that in order to improve the dephosphorization effect, it is necessary to create a slag environment with strong oxidation (WFeO controlled at 12% to 20%) and strong alkalinity (high CaO concentration, R value of 2 to 3), and ensure good slag fluidity. At the same time, maintain a moderately low temperature and strengthen the stirring of the slag to promote the dephosphorization reaction.

When the temperature of the molten steel reaches 1550℃, the oxidation period enters the second stage. This stage mainly carries out oxidation decarburization boiling refining, aiming to remove gas and inclusions in the molten steel. Our company uses ore combined with oxygen for decarburization. This reaction not only helps the molten pool to heat up quickly, but also helps to homogenize the composition of the molten steel. The specific chemical reactions are as follows: 0+C=CO, FeO+C=Fe+CO.

In the second stage of oxidation, ore should be added frequently and in small amounts, and stirring should be maintained to maintain active boiling of the molten pool and good fluidity of the slag.

In addition, the following points should be noted during the oxidation operation:

① Ensure that the oxidation and temperature measurement meet the requirements and the slag condition is good before adding ore in batches. The amount of each batch should not exceed 1% to 2% of the material weight, and the interval time between each batch should be greater than 5 minutes.

② In order to maintain good boiling of the molten pool, the oxidation decarburization rate should be controlled at 0.01% to 0.03% per minute.

③ Adjust the slag condition in a timely manner. At the beginning of oxidation boiling, the slag flow method is adopted to control the slag R value to 2 to 3, and the slag volume in the furnace is maintained at 3% to 4%. As the oxidation proceeds, it should be ensured that the slag has good fluidity and the slag layer is thin and uniform.

④ Strictly control the temperature. The entire oxidation period is a heating process, and the heating rate needs to be adjusted according to the phosphorus content in the molten steel. At the end of oxidation, the molten steel temperature should be raised to 10 to 20°C higher than the tapping temperature of the steel grade.

⑤ Natural boiling. When the temperature and chemical composition are suitable, stop adding ore and adjust the slag, let the molten pool boil naturally for 5 to 10 minutes to reduce the residual oxygen content in the molten steel and allow the gas and inclusions to float fully.

⑥ Timely slag removal. Since the slag in the oxidation period has a high FeO content and contains P2O5, in order to ensure the smooth progress of the reduction period and prevent phosphorus reversion, the slag removal operation must be carried out when the slag removal temperature is 10 to 20°C higher than the steel tapping temperature. At the same time, it should be ensured that carbon, phosphorus and other restrictive components meet the requirements.

⑦ Carbon addition operation. If the carbon content is too low at the end of oxidation, pure and dry carbon powder can be sprinkled on the exposed steel surface after slag removal for carbon addition.

VI. Reduction Period

After the oxidation period, the reduction period begins. The main task of this stage is to create a good reduction environment so that the molten steel can be deoxidized and desulfurized, and adjust its chemical composition to ensure that the temperature is suitable when tapping. The operation process of the reduction period is as follows:

After the power outage and slag removal, the first step is to add ferromanganese for pre-deoxidation. Subsequently, lime, fluorite and broken silica bricks should be added immediately to make a thin slag layer to reduce the contact and cooling of the molten steel with air. The addition ratio of these materials should be controlled at 4:1:1, and the total addition amount is about 2% to 3% of the weight of the molten steel.

After the thin slag is formed, sampling and analysis should be carried out immediately to detect the content of key elements such as C, Mn, Si, S, and P. Then, add an appropriate amount of reducing carbon powder and ensure that the furnace door is quickly closed to maintain the sealing of the furnace, thereby promoting the rapid formation of white slag.

As the reduction reaction continues, the deoxidation and desulfurization capacity of the slag will gradually weaken. Therefore, it is necessary to regularly replenish slag-making materials, adding a batch every 6 to 8 minutes to maintain the fluidity of the slag. At the end of the reduction period, ferrosilicon and ferrochrome should also be added to prepare for steel tapping.

In order to ensure sufficient deoxidation and desulfurization effects, the molten steel should be reduced for at least 15 minutes under the white slag cover and have good fluidity. The slag volume during the entire reduction period is controlled at 2% to 3% of the charge, and the ratio is lime: fluorite: carbon powder = 4:1.5:1.

When the oxygen and sulfur contents in the molten steel are reduced to the qualified standards, the molten steel temperature can be measured. Once the tapping temperature requirement is reached, the chemical composition of the molten steel is adjusted.

After the chemical composition and molten steel temperature are adjusted, the final deoxidation operation is carried out. The amount of aluminum added is controlled at 0.1% to 0.15% of the weight of the molten steel to ensure that the deoxidation effect meets the requirements.

VII. Steel Tapping

The following points should be noted during steel tapping:

First, ensure that the composition is qualified and the main elements are controlled within the internal control specification range;

Second, the deoxidation effect is good, the slag must be in a white slag state before adding ferrosilicon, and the steel tapping must be completed within 10 minutes;

In addition, the steel tapping trough must be kept clean, dry and flat, and aligned with the steel tapping port to facilitate smooth steel tapping;

Finally, the steel ladle needs to be baked to dark red in advance, and an appropriate amount of silicon, aluminum, barium and rare earth ferrosilicon should be added 15 minutes before steel tapping for final deoxidation.

After steel tapping, samples should be taken from the steel ladle to check the temperature and deoxidation of the molten steel. According to the temperature of the molten steel in the ladle, combined with the actual situation of the baking ladle and the amount of slag, the calming time is determined to ensure that the initial pouring temperature of the casting does not exceed the pouring temperature of the steel grade. Generally, at least 5 minutes of calming time should be ensured after steel tapping.

For ordinary electric arc furnaces, the use of molten oxygen combined technology helps to reduce the gas content and phosphorus content in the molten steel during the melting period. During the reduction period, the degassing and impurity removal process in the molten steel can be further promoted by adopting reduction refining technology. The application of this smelting process not only improves the quality of the molten steel, but also optimizes the organizational structure and mechanical properties of the steel castings, while also helping to improve the surface quality of the castings and bringing certain economic benefits.