Abstract: With the progress of continuous casting industry, steel works in the progress of the process equipment, billet continuous casting machine’s drawing speed is rising steadily. Shanxi Jianbang Industry Group for 160 * 160 small billet caster, in simple equipment modification and process optimization to improve, to achieve the HRB40OE in production process is drawing speed by 3.5 m/min up to the highest of 4.76 m/min, and can be stable at 4.5 m/min, after the speed increase of production line, casting internal and external quality is stable, continuous casting production capacity to effectively release.
The difficulty of high-speed continuous casting is that the billet can be produced stably under the condition of high drawing speed and its quality will not be affected.The change of drawing speed is an important parameter to control the continuous casting process, which changes the interaction between mold and billet. With the increase of drawing speed, the fluctuation of steel liquid level will be intensified continuously, and the friction resistance will increase significantly, finally leading to the bonding and cracking of billet and the occurrence of steel leakage .The improvement of drawing speed will lead to the loss of surface and internal quality of the billet, so the control of steel leakage and billet quality is of great importance to the improvement of continuous casting speed.
1.1 Basic parameters of casting machine
The steel mill of Tongcai Industry and Trade Co., Ltd. of Shanxi Jianbang Group has 3 top and bottom blowers of 65 T and 2 six-machine and six-flow billet continuous casting machines with a section of 160mm×160mm.Main production of steel for Q195, Q235, HPB300, HRB400E, HRB50OE, MG335, MG400, MG500, 30MnSi, etc.The basic design parameters of no. 1 continuous casting machine are shown in Table 1.
1.2 The link limiting the drawing speed of continuous casting
At present, HRB400E is the main type of steel for no. 1 continuous casting. Before the optimization and transformation of process equipment, the drawing rate of continuous casting can be basically stable 3.0 m/min – 3.3 m/min.Without changing the existing process and equipment, under the condition of continuous casting speed is greater than 3.3 m/min, after continuous caster breakout probability increase greatly, at the same time, the appearance of the casting billet quality and internal quality have fallen sharply, as the speed of ascension, casting billet low power display corner crack, crack between rating increased significantly, such as degree of slab appearance to take off the side also increased greatly.It seriously affects the stable and smooth production of steel rolling in the later process, and seriously restricts the production capacity of steel mill.Therefore, it is imperative to tackle the problem of high drawing speed of billet!
The main factors limiting the casting speed are the safe thickness of the mould outlet and the metallurgical length of the casting machine.The safety shell thickness of the mould outlet is also called the minimum shell thickness.In order to prevent the risk of steel leakage, the thickness of mould shell should not be less than 8 ~ 10mm.The growth of billet shell in continuous casting mould is approximately subject to solidification law :(figure 2)
Formula : K is the solidification coefficient. Generally, the solidification coefficient of billet is 20~26mm/min1/2 . L is the effective length of mold, and the length of mold copper tube is 0.9m.Delta is the thickness of the safety shell, take 10mm.
Therefore, in accordance with the guarantee of safe shell thickness at the mold outlet, when the mold cooling and solidification coefficient is set at 24mm/min’, the theoretical maximum drawing speed is 4.60m/min: when the condensation cooling and solidification coefficient is set at 2Emm /mi1/, the theoretical maximum drawing speed is 5m/min.
The metallurgical length of no. 1 continuous casting machine is set at 28m. The comprehensive solidification coefficient K of the casting billet is set at 28 ~ 32mm/min. Its calculation is subject to :(figure 3)
Formula :Le is the metallurgical length of the casting machine, m.Le=28m; D is the thickness of the casting billet, mm.D=160mm;K is the comprehensive solidification coefficient, mm/min1/2.The comprehensive solidification coefficient is usually between 28 and 32mm/min1/2.The solidification coefficient is set at 30mm/min1/2, then The maximum pulling speed of Vmax theory is 4.8m/min.
Combined with the safe thickness of the mould outlet blank shell and the maximum metallurgical length of the present equipment, the maximum drawing speed of the current equipment can reach 4.8m/min.Therefore, how to improve the cooling intensity of the mold to ensure that the cooling coefficient of the mold can reach 25mm/min1/2, and the comprehensive solidification coefficient can reach 30mm/min1/2 is the key to ensure high drawing speed.Therefore, the solidification coefficient of the process can be improved mainly through the following improvements, so as to ensure the normal operation of continuous casting with high drawing speed.
2.1 Optimization of mold parameter form
The mold is commonly known as the heart of the continuous casting machine. Its rational design and manufacturing quality play a vital role in the production of the continuous casting machine and the quality of the casting billet.Before modification, the mould adopts tube type mould, with the water seam width of 3.5mm, parabolic taper and the difference between upper and lower orifices of about 1.6mm.With the development of high tensile test, the degree of mould in the middle and later period (about 4000 tons of steel passing) is obviously intensified.Therefore, the mold is optimized.
2.1.1 Add 0 sufficient mixing
In the early production process, due to the low drawing speed, low deformation of casting billet and low rate of steel leakage, the necessity of upper foot rod is not very great, and it is more difficult to deal with the mould with foot wheel mould in the accident of steel leakage of mould outlet.Therefore, the previous production process was to install the foot stick.With the experiment of high drawing speed, the deformation rate of the cast billet increases and the risk of steel leakage rises after the mould outlet. Therefore, the increase of 0 section of foot rod plays a supporting and guiding role on the mould outlet billet, that is, it plays a supporting role on the weak shell of the steel pipe at the beginning, reducing the deformation or steel leakage of the cast billet.
2.1.2 Optimize the taper of the mold
The contact between the shell and the mould in the direction of the length of the mould facilitates the uniform growth of the shell in the mould.This prevents excessive increase of friction in the mould, especially in the lower part.Before the production process of parabolic mold cannot satisfy the production of the high speed, high speed production process considerably increase the accident rate, so the form of crystallizer is replace by the parabola taper for diamonds, at the same time increase the taper, poor mouth up to 1.8 mm from top to bottom, to improve the whole crystallization at the same time in order to reduce friction, the 300-400 – mm from the top of crystallizer, until the mouth mould no taper Angle area, and the down, no taper Angle area.This not only ensures the uniform growth of the mould shell, but also effectively prevents the excessive increase of friction in the mould, especially in the lower part.
2.1.3 Increase the mold water flow
With the increase of the drawing speed, the heat flux of the crystallizer increases obviously, and the temperature difference between the inlet and outlet water of the crystallizer rises to more than 9℃. In order to control the temperature difference between the crystallizer water and prevent the local boiling of the crystallizer copper wall, the flow rate of the crystallizer is increased appropriately from 145t/h to 155t/h, and the process ensures that the temperature difference between the inlet and outlet water of the crystallizer is controlled within 7-9℃.
2.1.4 Mold vibration optimization
At high drawing speed, the vibration parameters should be selected to ensure a certain negative slippage time so that the billet can be removed smoothly, and a certain positive slippage time should be ensured at the same time to improve the consumption of the protection slag and increase the lubrication function.Reasonable or optimal process parameters are realized by the correct selection of basic parameters.The basic parameters of sinusoidal vibration, amplitude and frequency, should be determined on the premise of obtaining the best technological parameters, so as to ensure the smooth demodulation and good quality of the billet.At present, the on-line frequency of vibration is 220C/min, with the maximum amplitude of 5.6mm.Figure 4 is obtained by calculation.
According to relevant materials and data from manufacturers, the value range of negative sliding time is 0.1-0.25s, and the best negative sliding time for different types of steel is about 0.1s.Domestic and foreign literatures reported that the NS value changed in the range of -240%-20% for casting, and the results did not have any adverse effect on casting billet demoulding and surface quality.NS value of amplitude 5.6mm and frequency F = 60V :-34.4%-0.57%.After comprehensive consideration, the basic vibration parameters were selected as amplitude 5.6mm and frequency F = 60V, with the maximum vibration frequency of 220C/min, which could meet the process demand of about 4.5m /min high drawing speed.
2.3 Optimization of secondary cooling section
In order to adapt to the high drawing speed, increase the cooling effect and improve the cooling and solidification coefficient of the process, add one section or four sections after the original third section, and change the jet figures of each section, as shown in Figure 5 below.
The maximum effluent total of the first two cooling stages changed by the nozzle was 71.4t/h, and after the change, the maximum effluent total of the second cooling stage was 102.2t/h, with an increase of 30.8t/h.
2.4 Optimization of secondary cooling water distribution
For continuous casting Secondary cooling is to force uniform cooling on the surface of the billet, so that the billet can control cooling and solidification in a relatively short time, and reduce the phenomenon of steel leakage in the shell is too thin.Before optimization, the unreasonable proportion of each section of the secondary cooling water distribution used in No. 1 continuous casting resulted in high intermediate crack rating of the billet.After the optimization of the water distribution curve, the inner quality of the billet was effectively improved, all meeting the requirements (FIG. 6).
Effect after transformation
1) The drawing speed of the casting machine is obviously improved.After the transformation and optimization of No. 1 continuous casting machine, after three months of production practice, the drawing speed can be stable at 4.3m /min if the production rhythm allows, and the maximum drawing speed reaches 4.7m /min.
2) The production capacity of steel mills has been released.The daily output of single stand (6 machines and 6 streams) reached 3391.15 tons, and the output reached 282.60 tons.
3) Reduce the outlet temperature of the converter.With the improvement of the drawing speed, the steel outlet temperature of the converter has been greatly reduced. The average steel outlet temperature from June to September 2020 has been reduced from 1670 ° C previously to 1630 ° C now, effectively supporting the work of reducing cost and increasing efficiency of the steelmaking plant.
4) The low quality of continuous casting is normal, and there is no obvious out-of-square phenomenon. The casting billet can be used in the next process.
The average drawing speed of continuous casting has been raised from the original 3.5m /min to 4.3m /min, and the maximum drawing speed has reached 4.7m /min by optimizing and adjusting the equipment process of the billet continuous casting machine in our factory, mainly by adjusting the mold parameter form, optimizing the protection slag and changing the secondary cooling section.At the same time release steelmaking capacity, with a daily capacity of 13,000 tons (two 6-machine 6-stream continuous casting machines).The average output temperature of the converter is reduced by nearly 50℃, which effectively supports the work of reducing cost and increasing efficiency of the steelmaking plant, and meanwhile, the quality of the billet is stable and controllable.