Discussion on ways to increase the temperature of continuous casting billet in continuous casting and direct rolling process

The temperature of the continuous casting billet is the prerequisite and guarantee for the smooth implementation of the continuous casting and straight rolling process. Various methods of increasing the temperature of the continuous casting billet are analyzed, and the continuous casting-rolling operation using supplementary heating and reasonable settings of the continuous casting billet is analyzed. Measures such as rhythm are also discussed, and the future technological development direction of increasing the temperature of continuous casting slabs is prospected.

Keywords: continuous casting and direct rolling; continuous casting billet; temperature; casting speed; water volume

Continuous casting and direct rolling is one of the hot charging processes of continuous casting. It means that the temperature of the continuous casting billet is above 1100°C, eliminating the heating process of the heating furnace and directly rolling. Compared with other continuous casting hot transfer and hot charging processes, the continuous casting and direct rolling process omits the heating process of the heating furnace and is more difficult to implement, but its comprehensive economic benefits are also the highest. Under the continuous casting and direct rolling process conditions, the billet required for rolling steel is directly provided by the continuous casting machine, which requires that the cast billet must meet the rolling temperature requirements when it is sent to the rolling mill. One of the important factors in realizing this technology is to increase the temperature of the continuous casting billet.

Advantages of continuous casting and direct rolling process

In the energy consumption of the hot rolling process, fuel consumption accounts for more than 70% of the total energy consumption, and fuel consumption is only generated in the heating process of the heating furnace. Therefore, energy saving of the heating furnace is the key to energy saving in the rolling process. Compared with traditional cold charging, the continuous casting and direct rolling process eliminates the heating process of the heating furnace and utilizes the physical heat energy of the continuous casting billet itself, which not only reduces fuel consumption and pollutant emissions, but also saves the time spent on maintaining the heating furnace. required maintenance costs.

Taking a bar and wire rod workshop with an annual output of 1 million tons as an example, comparing the energy consumption of traditional cold charging and continuous casting and direct rolling heating furnaces, canceling the heating furnace and adopting direct rolling can save 45,000 tons of standard coal, which can save 45 million yuan per year. Direct rolling avoids secondary heating of the billet and reduces oxidation loss by at least 1.5%. Calculated at a unit price of 3,000 yuan/t, it can save 45 million yuan per year. Comparative data on the management and maintenance costs of traditional cold charging and continuous casting and direct rolling heating furnaces show that the use of continuous casting and direct rolling eliminates the heating process of the heating furnace, saving 3.25 million yuan in heating furnace management and maintenance costs throughout the year. To sum up, for a rod and wire workshop with an annual output of 1 million tons, the annual benefit of 93.25 million yuan can be obtained by canceling the heating process of the furnace. The comparison of pollutant emissions before and after the heating process of the heating furnace is cancelled. In terms of indicators, continuous casting and direct rolling can achieve zero emissions of relevant pollutants. Energy consumption, management and maintenance costs and pollutant emission data are shown in Table 1. In summary, it can be seen that the promotion of continuous casting and direct rolling technology can bring great economic and environmental benefits.

 

Continuous casting machine, continuous casting billet
Continuous casting machine >>

 

Table 1 Comparison of process parameters between cold loading and continuous casting and direct rolling

Process name  Blank temperature/℃  Furnace heating time/h   Fuel consumption/10,000 tons of standard coal Oxidation burning loss/10,000 tons Maintenance cost/10,000 yuan Labor cost/10,000 yuan Mechanical power consumption cost/10,000 yuan Carbon dioxide/10,000 tons Carbon emissions/10,000 tons  Dust/10,000 tons Sulfur dioxide/10,000 tons 10,000 nitrogen oxides/10,000 tons
Cold installation room temperature 4 4.5 1.5    75 100 150 11.21 3.06 3.06 0.34 0.19
Continuous casting and direct rolling>1100 0 0 0    0 0 0 0 0 0 0 0

Ways to increase the temperature of continuous casting billet in continuous casting and direct rolling process

When designing the steel plant, the original continuous caster and rolling workshop did not consider the hot-feed function of direct rolling. Therefore, systematic optimization is required during the later transformation of the continuous casting and direct rolling process. The temperature of the continuous cast billet is mainly affected by the billet exit temperature and the temperature drop during transportation to the rolling mill after being cut to length. Therefore, the main measures to increase the temperature of the continuous cast billet in engineering are to increase the billet exit temperature and reduce the temperature drop.

Increase the continuous casting speed

Increasing the continuous casting speed can reduce the cooling residence time of the slab in the crystallizer and secondary cooling zone, and the temperature of the slab will increase. The drawing speed of domestic billet continuous casting is generally 2 to 3 m/min, while the drawing speed of continuous casting in the internationally developed endless rolling technology is 6 to 8 m/min; therefore, the continuous casting of billet There is still a lot of room for improvement in the casting speed.

In the actual production process, the continuous casting speed is also limited by the steelmaking output. The casting speed of each stream can be increased by reducing the number of pouring flows. During billet continuous casting, the number of pouring flows is reduced from n to n-1, the pulling speeds before and after speed increase are V1 and V2 respectively, and the cross-sectional area of the billet is A. Calculated with reference to the law of conservation of mass, after reducing the number of pouring flows from n to n-1, the pulling speed can be increased by n/(n-1) times.

Optimize the water volume in the secondary cooling zone

After the slab comes out of the crystallizer, a shell of a certain thickness will be formed. The function of the secondary cooling zone is to continuously cool down the cast slab through cooling water.

Cooling makes it gradually complete solidification, so the water distribution in the secondary cooling zone has an important impact on the temperature of the slab. Figure 1 shows the effect of the total water volume in the secondary cooling zone on the surface temperature of the slab at the cutting point. As can be seen from Figure 1, reducing the amount of water in the secondary cooling zone can increase the temperature of the slab. However, if the amount of water in the secondary cooling zone is too small, steel leakage may occur, so the temperature of the slab needs to be automatically controlled.

Figure 1  Effect of the amount of cold water on the surface temperature of the slab

1) First, use finite element simulation to conduct finite element analysis of the temperature field of continuous casting production. Regression is used to calculate the mathematical model of the influence of secondary cooling water volume and other influencing factors on the slab surface temperature and solidification end temperature.

2) The slab temperature automatic control system is equipped with an online thermometer and can read the casting speed, pouring temperature, secondary cooling water volume and other process parameters in real time. Based on the real-time measured process parameters and the corresponding mathematical model, the The amount of secondary cold water is adjusted in real time. The actual measured temperature is lower than the preset temperature, and the surface temperature of the slab is too low, which cannot meet the requirements of continuous rolling. It is necessary to appropriately reduce the amount of secondary cooling water according to the mathematical model. If the actual measured temperature is greater than the preset temperature and the surface temperature of the slab is too high, a steel leakage accident may occur, and the amount of secondary cooling water needs to be increased according to the mathematical model.

 

The continuous casting billet is cut using hydraulic shears

Traditional continuous casting billet cutting uses flame cutting, and the time to cut a billet is 30 s. However, with hydraulic shears, the entire cutting process is about 8 s, which is much lower than the 30 s of the flame cutting machine. This gives the continuous casting billet Reduces temperature loss by at least 25°C. With the continuous advancement of hydraulic shear design and manufacturing technology and its active application in practice, the operation quality of the equipment has been greatly improved. When replacing the continuous casting tundish, the hydraulic shear can be replaced online, which will have less impact on production. However, no matter what type of hydraulic shear is used, the quality of the sheared blank end face is not as good as that of the flame cutting machine. When choosing the endless rolling process, you need to choose carefully.

 Rapid conveyor roller

The temperature drop rate after the slab is cut to length is about 25°C/min. For the existing bar and wire rod production line, due to the long relative plane distance between the continuous casting workshop and the steel rolling workshop, the slab is usually transferred through roller tables and skyscrapers. Cars, span trolleys and special slab transport and insulation cars are transported to the steel rolling workshop for production. When transforming the continuous casting and direct rolling process, it is difficult to change the relative positions of the continuous casting machine and the rolling mill, and it has a wide impact. Therefore, a fast roller table is added to send the cast slabs cut to length directly from the continuous casting machine to the roughening machine. The rolling mill unit has become the first choice for the transformation of continuous casting and direct rolling processes. Even if the distance between the continuous casting machine and the rolling mill is 100 m, after the fast roller speed reaches 3 to 5 m/s, the billet transfer between the continuous casting machine and the rolling mill can be completed in about 30 s.

Before cutting, the slow travel time is generally >5 minutes due to the influence of the continuous casting speed. Therefore, the temperature drop during the rapid conveyor roller transfer process is 10 times smaller than before cutting the billet.

Configure heat preservation roller table

For a billet with a length of 12 m, under the condition of a pulling speed of 3 m/min, it takes 4 minutes to reach the fixed length position from the cutting position; if no insulation measures are taken, the temperature of the billet will drop to approximately 100°C in 4 minutes. The temperature drops especially at the corners are the most severe. Without special insulation measures, the temperature will be unstable. Relevant companies use advanced insulation rollers to prevent the rollers from taking away the temperature of the billet. At the same time, it can be ensured that the surface temperature of the slab before entering the continuous casting cooling bed is still above 1000°C. During the transfer process, the cast slab mainly loses heat to the surrounding environment in the form of thermal radiation. Covering the roller table with an insulation cover can effectively reduce the heat loss due to radiation. The temperature inside the insulation cover is much higher than room temperature, which reduces the risk of casting. The temperature difference between the billet and the radiation environment reduces the radiated heat. Research shows that adding an insulation cover can reduce radiation loss by about 25%.

Supplementary heating of continuous casting billet

During the continuous casting and straight rolling process, the induction furnace can be used to inductively heat the corners of the billet to increase output and production efficiency. At the same time, for enterprises that have the conditions to use blast furnace gas and other chemical gases, tunnel heating furnaces can be used for supplementary heat.

 Set a reasonable operating rhythm for the continuous casting and rolling areas

Multi-stream continuous casting, corresponding to one straight rolling line or multiple straight rolling lines, should be equipped with automatic detection and sorting functions, and the sequential steel rolling can be automatically adjusted according to the position and temperature of each stream to ensure the reliability of the process. At the same time, the continuous casting billet transfer process is optimized and designed using queuing theory to reduce the equipment vacancy rate and ensure that the time interval and temperature drop of the billet after being discharged from the continuous casting machine and entering the rolling mill is short and the temperature drop is small.

The lower-temperature cast slabs are no longer handled on the ground. During the process, they are directly downgraded and transferred to the next process. This reduces the frequency of processing low-temperature steel and ensures the effectiveness of high-temperature billet delivery. Calculate the margin load of the roughing mill unit of the rolling line and reasonably allocate the pass reduction, so that the rolling line can smoothly roll low-temperature continuous casting billets and form integrated steelmaking-rolling production management.

Summary and outlook

As one of the most promising steel production processes in the future, the temperature of continuous casting and direct rolling is an important factor for reliable implementation. Increasing the temperature of continuous casting is an important prerequisite and guarantee for promoting its widespread application. The temperature of continuous casting billet is mainly directly affected by the billet temperature and temperature drop. At present, a variety of methods to increase the temperature of continuous casting slabs have been proposed, but the current research on the impact of continuous casting slab temperature is only a simple measure to increase the temperature of the slab and reduce the temperature drop of the slab. For more rigorous and sophisticated theories, There is little research on the basis and use of advanced automated equipment to achieve high-efficiency hot billet direct rolling; and there is less research on equipment for heating of continuous casting billets, which has not been systemized and cannot be effectively applied in actual production. Continuous casting and direct rolling are widely used in various steel plants, and a lot of work still needs to be done: 1) Develop an efficient continuous casting process and try to increase the temperature of the continuous casting billet while ensuring safe production. 2) Optimize the layout of the newly designed steel plant to provide conditions for the continuous casting and direct rolling process. 3) Realize integrated and refined management of steel making and rolling, and implement a linkage mechanism between the two plants to ensure the stable operation of production. 4) The continuous casting billet heating technology and equipment should be strengthened and developed.

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