This paper discusses the methods and ways to produce more steel with low iron consumption steelmaking process when iron water resources are small, explores and studies the low iron consumption all-iron steelmaking process, and effectively solves the problem of heat shortage and low iron consumption steelmaking process. Low-temperature slag splashing problem in all-iron steelmaking.
Keywords :steelmaking; converter; all-iron steelmaking; low iron consumption; temperature; slag-retaining operation
A converter plant currently has one 900t mixing furnace, two 65t converters, one 65t LF refining furnace, one 5-machine 5-strand billet continuous caster and one 4-machine 4-strand billet continuous caster. The main steel types produced are: 20MnSi, 20MnSiNb, Q235, 45#, 60Si2Mn, billet section: 150mmx150mm, 160mm×160mm. The annual steel production in 2006 was 1.75 million tons, the planned steel production in 2007 is 2.2 million tons, and the iron water resources are expected to be 1.7 million tons. Therefore, the key to completing this year’s steel production plan is to further reduce the consumption of steelmaking water.
Low iron consumption steelmaking process
Converter steelmaking is carried out by utilizing the physical heat and chemical heat of molten iron. The temperature in the furnace depends on the heat income and heat expenditure of the charge. Generally speaking, the temperature control of converter smelting mainly lies in reasonably controlling the heat expenditure. Smelting temperature control has an important impact on the smooth progress of the smelting process, consumption of metal materials, steel quality, furnace lining life and continuous casting. Therefore, the idea of low iron consumption steelmaking process is: change the structure of the furnace charge, steadily increase the physical heat of molten iron, Reduce the amount of lime, lower the tapping temperature, and add an alloy baking device.
Change the charge structure and use all-iron steelmaking
The so-called all-iron steelmaking means that most of the coolant used for steelmaking uses pig iron blocks, and the furnace charge structure is mainly molten iron and pig iron blocks for steelmaking. The advantage is that the content of C, Mn, Si, and P heating elements in the charge structure is high, and the chemical heat contained is large, which is beneficial to the heat balance of the converter. It has been calculated that the thermal effect required for each kilogram of pig iron to melt and heat up to the tapping temperature (1670°C) is 1496 kJ/kg, of which the chemical heat stored in it is 982kJ/kg, and for each kilogram of scrap steel to melt and heat up to the tapping temperature (1670°C) it requires The thermal effect is 1443kJ/kg. When the molten iron consumption reaches 765kg/t, the pig iron consumption is increased to 339kg/t, and the scrap steel consumption is reduced to 13.07 kg/t. The disadvantage is that the low-temperature slag is thin in the early stage of the smelting process and is easy to splash.
Increase the temperature of molten iron entering the furnace
The composition of molten iron is shown in Table 1.
The physical heat of molten iron accounts for 50% of the heat income of the converter. Increasing the physical heat of molten iron is an important method to supplement the heat in the furnace. Nanjing Iron & Steel Converter Mixing Furnace has always been operated at low temperature. The temperature of the molten iron supplied by the mixing furnace is 1210~1270℃, and the average molten iron temperature is 1250℃. By increasing the temperature of the mixing furnace furnace, the molten iron entering the factory is quickly poured into the mixing furnace. , the temperature of the molten iron tapped from the mixing furnace has been increased to 1280~1350°C, and the average molten iron temperature is 1300°C. The effect of increasing the temperature of the molten iron is obvious. When the temperature of molten iron increases by 50°C, the physical heat per kilogram of molten iron increases:
Cs—solid heat capacity of pig iron, kJ/kg·K;
Tt—melting point of molten iron, ℃;
C—liquid heat capacity of molten iron, kJ/kg ·K;
Tiron—temperature of molten iron, ℃.
Tt unified melting point of pure iron, ℃;
G—Element content in molten iron, %;
K-The value by which elements dissolved in iron reduce the melting point of iron.
The melting point of molten iron is calculated to be 1071°C. The physical heat increase per kilogram of molten iron increased by 50°C is Qiron = 9.99×4.186kJ/kg.
Optimize the slagging system
Low iron consumption, all-iron smelting, because the amount of pig iron blocks is large, if the slag-making system is unreasonable during the smelting operation, it is easy to cause the amount of slag to be too large, the physical heat of the slag in the heat expenditure item is increased, and the “back-drying” in the smelting process is serious, which is prone to occur Due to the large blowout, part of the pig iron at the bottom of the furnace at the end was not completely melted, and the molten steel was churning and it was difficult to turn over the furnace, so the slag-making system was changed.
The slag retention operation refers to leaving part or all of the slag in the furnace after splashing. The slag retention operation can quickly form slag with good fluidity, high alkalinity and high FeO at a lower temperature in the early stage of smelting, thereby optimizing the early dephosphorization effect, improving the slag removal effect and utilization rate of lime, reducing lime consumption and The consumption of iron in the slag reduces the heat expenditure taken away by the slag. At the same time, the temperature of the slag after splashing is around 1550°C, which supplements a certain amount of physical heat for the next furnace of smelting. When the Si content of molten iron is ≤0.80%, the slag content can account for 50% of the total slag content. The physical heat Qr brought by each kilogram of slag content is:
Qslag= C₁· (Tslag-25)+209
C—Slag liquid heat capacity, kJ/kg·K;
Tslag—the slag temperature, ℃.
Physical heat brought by each kilogram of slag left Qslag =504.58x 4.186 kJ/kg
Use light burnt dolomite
The components of lime and light-burned dolomite are shown in Table 2 and Table 3.
Lightly burned dolomite can replace part of the lime in slag formation, which is beneficial to early slag formation, promotes lime dissolution, assists slag formation, and increases the MgO content in the slag. To weaken the role of slag in lining erosion and protecting the furnace lining, the steel consumption per ton is now controlled at 15.09kg, and the lime consumption has dropped significantly, from 63kg/t to 40kg/t. The decomposition heat effect of CaC₃O is 1690×4.186 kJ/kg, and the decomposition heat effect of MgCO₃ is 1405×4.186 kJ/kg. It is calculated that the heat effect of light burnt dolomite per kilogram is 175.5×4.186 kJ/kg less than that of lime per kilogram.
Take the low-temperature route
The low-temperature route aims to reduce the tapping temperature by reducing the process temperature drop. Reducing the tapping temperature is conducive to the implementation of a low-iron consumption all-iron smelting process (1) Add an alloy baking device. After the alloy is baked for 2 hours, the temperature can reach 200°C, which reduces the temperature drop caused by the melting of the iron alloy during the tapping process.
(2) Ensure that the ladle is baked online, red envelopes are tapped, and the number of running ladles is reasonably controlled to 6.
(3) Straighten out the production order, speed up the production rhythm, reduce the time for pouring molten steel, and control the waiting time for pouring to 8 minutes, and strive to ≤ 6 minutes.
(4) Continuous casting implements full-process protective casting, lowers the temperature of the tundish, and strengthens the baking of the tundish. The baking time should not be less than 90 minutes. The liquid level of the tundish should be filled with sufficient covering agent to reduce the temperature drop rate of the ladle.
By improving the level of production management and implementing the above measures, the tapping temperature has gradually dropped, 10°C lower than the average tapping temperature of 1,687°C in 2005. Reduce molten steel in heat expenditure
Physical heat, physical heat reduction per kilogram of molten steel Qstccl:
Qstccl x=Qstccl-Qstccl after
Cs—solid heat capacity of steel, kJ/kg·K;
Ts—melting point of molten steel, ℃;
C—Liquid heat capacity of molten steel, kJ/kg·K.
T is determined according to the method of calculating the melting point of molten iron. The end point of molten steel is C: 0.08%, Mn: 0.09%, P: 0.024%, S: 0.024%
The calculated Tg is 1523.03℃. The physical heat reduction of each kilogram of molten steel is Qstccl x=1.997×4.186 kJ/kg.
Operating gun position control
Low iron consumption, all-iron smelting, adopts a slag-retaining operation process. The initial slag contains high (FeO) (SiO). To ensure that the (FeO) in the slag does not cause excessive low-temperature splashing in the early stage, the gun position in the early stage of smelting is The gun position is 100mm lower than the normal gun position. In order to ensure that the pig iron block is completely melted in time, the oxygen flow rate is controlled to 17000m³/h before slagging, and the feeding time is delayed by about 1 minute.
|Heat income changes||Change in heat expenditure × 91% (heat loss 9%)|
|Iron consumption dropped from 858.88 to 76, heat reduction||pig iron lump increaseAdd 118 kg/t instead of 65.78 kg/ Scrap heat expenditure reduced||Lime consumption reduced by 22.731kg/t heat expenditure reduction||Residue amount19.73 kg/t heat supplement||The temperature of the molten iron increases by 50°C.||Tapping temperature reduces heat consumption by 10°C and reduces expenditure|
Establishment of loading system
Quantitative loading is used during converter smelting, with a total loading of 84t, a steel tapping volume of (75±0.5) t, iron consumption controlled at 760~800kg/t (i.e. molten iron (58±1.5) t), and cold material exceeding 95%. Pig iron (i.e. (25±1.5)t), and a small amount of scrap steel (i.e. (1±1)t) for others. The iron consumption control amount is determined according to the temperature of the molten iron and the Si content of the molten iron component, see Table 5.
|Molten iron temperature ℃||Si content in molten iron %||Iron consumption kg/t|
The converter adopts low-iron consumption all-iron smelting and increases the temperature of the molten iron entering the furnace. During the smelting process, light-burned dolomite is used to replace part of the lime to make slagging. The slag-retaining operation method is adopted and the low-temperature route is adopted to reduce the tapping temperature. Obvious results have been achieved in reducing iron consumption. , the limited amount of molten iron is utilized to the maximum extent, and the output increases month by month. In 2006, the iron consumption, output, and lime consumption of the converter plant are shown in Figure 1 and Figure 2.