Study on Heating Time of Steel Coil in HPH Full Hydrogen Bell Annealing Furnace

Since its introduction into China, the all hydrogen hood type annealing furnace has been widely used in cold rolling enterprises due to its high production efficiency, good surface quality of annealed products, and low medium and energy consumption. Baosteel has introduced a total of 60 fully hydrogen bell type annealing furnaces, of which 48 are HPH bell type furnaces from Germany's LOI company. From the current usage of the HPH bell furnace by Baosteel, it can be seen that Baosteel has not fully mastered the control model technology of the HPH bell furnace. The independent development of the annealing system mainly relies on empirical settings, which lack adaptability to complex and variable actual production. Optimizing the annealing system, especially controlling the heating time of the steel coil (the heating stage of the steel coil includes two processes: heating and homogenization, and the time when the lowest temperature inside the steel coil reaches the process required temperature is the steel coil annealing heating time), is of great significance for improving production efficiency and product quality. Therefore, it is necessary to study the influence of production parameter changes on the heating time of the steel coil through numerical simulation methods.
Many references 11-72 have introduced the heat transfer process inside the full hydrogen hood furnace, established and verified the mathematical model of heat transfer in steel coils, and based on this, reference 182 analyzed the relationship between the circulating air volume and annealing time inside the hood furnace. This article comprehensively discusses the changes in production factors that affect the heating time of steel coils during the operation of Baosteel's HPH bell furnace.
Mathematical model of heat transfer in 2 steel coils
Establishment of Heat Transfer Model for Steel Coil 21
Based on the heat transfer situation of the steel coil on site, the following simplified assumptions can be made:
(1) The steel coil structure is symmetrical, and the circumferential heat conduction can be ignored. Only the radial and axial heat transfer of the steel coil is considered, and its heat conduction equation is simplified to two-dimensional unsteady heat conduction:
(2) Steel coils are made of thousands of layers of strip steel, and their radial heat transfer is achieved through the comprehensive effect of heat transfer in the air gap between adjacent layers of strip steel, including heat conduction through protective gas, radiation heat transfer between strip steel, and heat conduction through contact points. In the model, the radial equivalent thermal conductivity coefficients of 17, 9, and 102 are used to characterize the radial thermal conductivity of steel coils
(3) The steel coil has no internal heat source and its heat exchange with the outside meets the third type of boundary condition.
The thermal conductivity equation of steel coil and its initial and boundary conditions are 152:
In the formula: r is the radial coordinate of the steel coil/m; z is the axial coordinate of the steel coil/m; Density of Q-force steel/kg # m'3;
CP is the specific heat capacity of steel/J (kge) -1;
Kcff is the radial equivalent thermal conductivity of the steel coil divided by W (m e); K is the thermal conductivity of the strip steel/W (me)'1; S is time/s; T is the temperature of the steel coil/e, Tg is the temperature of the protective gas/e, qR. is the heat flux density of radiation heat transfer between the outer surface of the steel coil and the inner cover/W # m'2: Ri is the inner diameter of the steel coil/m; Ro is the outer diameter of the steel coil/m; W is the height of the steel coil/m; HR, hR, ho, hw are the convective heat transfer coefficients/W (m2e 1'1) of the inner and outer, upper and lower surfaces of the steel coil, respectively.
212 Model Accuracy Verification
In Baosteel's production, a temperature measuring thermocouple is placed on the hood furnace platform to obtain the reference temperature at the bottom of the steel coil and control the time for the steel coil to be taken out of the furnace. This thermocouple records the temperature changes of the bottom point of the steel coil during the entire annealing process, providing a basis for us to verify the accuracy of the calculation model. In model validation, the calculated temperature value at that point is compared with the thermocouple test value. If the two are close, it indicates that the model is accurate. Plot the calculated temperature curve of the point together with the measured temperature curve in Figure 1, and list the parameters of the steel coil in Table 1.
The four curves in Figure 1 are the circulating gas temperature curve, the bottom calculated temperature curve, the bottom measured temperature curve, and the cold spot temperature curve. It can be seen from the figure that the calculated temperature curve at the bottom of the steel coil is very close to the measured temperature curve, and at the end of the insulation section, the temperature difference between the two is less than Se. This indicates that the computational model has high accuracy and can serve as a basis for further research. Additionally, it should be noted that when discussing a certain parameter, it is assumed that other parameter conditions remain unchanged, as shown in Table 1.
The structural parameters of steel coils include the radial equivalent thermal conductivity, outer diameter, and height of the coils. In actual production, the structural parameters of steel coils often change according to user needs, which affects the heating time of the coils.
31 1 Radial equivalent thermal conductivity
During the annealing process, the thermal conductivity of the steel coil itself includes axial and radial thermal conductivity. As mentioned earlier, the radial thermal conductivity of the steel coil is a comprehensive heat transfer process. The radial thermal conductivity coefficient is used to characterize the radial thermal conductivity of the steel coil, covering many factors closely related to radial thermal conductivity, such as steel plate thickness, rolling stress, etc.
By comparison, as the equivalent thermal conductivity increases, the time required for the annealed steel coil to reach the required cold point temperature (620 e 1) becomes shorter and the final temperature reached by the cold point also becomes higher. From Figure 2, it can be more intuitively seen the degree of influence of radial equivalent thermal conductivity on heating time. When the thermal conductivity increases from 414W/(ml e 1) to 1214W/(ml e), the annealing heating time decreases from 2219h to 1916h, shortening 313h. Observe the course of the curve
The potential curve changes from steep to gentle: the thermal conductivity increases from 414W/(ml e) to 614W/(ml e) and the proportion of 'increases from 1014W/(ml e) to 1214W/(ml e). Compared with the increase of 2W/(ml e), the annealing time of the former is shortened by 116h, while the latter is only shortened by 013h. It can be seen that when the radial equivalent thermal conductivity is relatively small, increasing its value is more conducive to shortening the heating time.
31 2 steel coil outer diameter
Radial heat conduction of steel coil is the process in which the heat obtained from the inner and outer surfaces of the coil is transferred to the cold point. When the radial equivalent thermal conductivity remains constant, increasing the outer diameter of the steel coil is equivalent to increasing the radial thermal resistance of the steel coil, so the time for heat transfer to the cold point is inevitably prolonged.
As the outer diameter of the steel coil increases. The rate of heating and cooling of the cold spot slows down during the annealing process. Figure 3 shows the heating time at different outer diameters. When the outer diameter of the steel coil increases from 116m to 214m, the annealing heating time of the steel coil increases from 1718h to 211711, extending by 319h. From Figure 3, it can also be seen that when the outer diameter of the steel coil changes from 116m to 210m, the slope of the curve becomes larger, and then the slope decreases. This is because as the outer diameter increases, on the one hand, the radial thermal resistance increases
Increasing and reducing the radial heat transfer of the steel coil: on the other hand, the cross-sectional area of the protective gas circulation channel between the outer surface of the steel coil and the inner surface of the inner cover decreases, resulting in an increase in gas circulation velocity, thereby increasing the convective heat transfer between the inner and outer surfaces of the steel coil. These two effects cancel each other out. Therefore, within the allowable outer diameter range of the furnace platform, increasing the outer diameter of the steel coil will prolong the heating time, but as the outer diameter increases, its impact on the heating time becomes smaller and smaller.
31 3 steel coil height
The axial heat conduction of steel coil is the process of transferring heat from the upper and lower surfaces of the coil to the cold point. The axial thermal conductivity of a steel coil is equal to the thermal conductivity of the steel grade, and the axial thermal conductivity of the steel coil is a constant value for a certain steel grade. As the width of the rolled steel strip increases, the height of the steel coil also increases, which is equivalent to increasing the axial thermal resistance while keeping the thermal conductivity constant. This inevitably prolongs the heating time for annealing the steel coil.
As the height of the steel coil increases, its annealing heating time is prolonged. In addition, the effect of changes in the height of steel coils on annealing heating time is more significant than that of radial equivalent thermal conductivity and changes in the external curvature of steel coils on heating time. Therefore, in production, special attention should be paid to adjusting the annealing process according to the changes in the width of the strip steel. Figure 4 shows the variation of annealing heating time with increasing coil height. When the coil height increases from 019m to 115m, the annealing heating time increases from 1513h to 2112h, which is an extension of 519h. From the slope of the curve, it can be seen that the annealing heating time is almost linearly related to the height of the steel coil, which also indicates that changes in the height of the steel coil have a significant impact on the heating time.
By applying the above rules, we have revised Baosteel's online production annealing time control model, improved the accuracy of the model calculation, not only significantly reduced human intervention in the control process during production, but also improved product quality.
4. The impact of changes in operating parameters on heating time
412 circulating gas temperature
During the production process, different circulating gas temperature regimes should be selected for annealing production according to the steel grade and user requirements. The selection of temperature system is directly related to the quality of the product. If the annealing temperature is too high, the steel coil is prone to bonding; if the annealing temperature is too low, it is also prone to rolling hardening. In Baosteel's actual production, these two issues are also the main reasons for the generation of waste products. In addition, during on-site production, due to burner malfunctions, the temperature of the circulating gas is often lower than the set temperature, resulting in an extended time for steel coils to be taken out of the furnace. Therefore, it is necessary to discuss the effect of temperature changes in the circulating gas on the annealing heating time.
In the early stage of the heating stage, the increase or decrease of gas temperature has little effect on the cold spot temperature, and then the influence of gas temperature on the cold spot temperature increases significantly. Figure 7 shows the difference in heating time required to achieve the required cold spot temperature for the process under five temperature regimes. Under the actual production temperature system, it takes 1910 hours for the cold point to reach 620 e. If the temperature system is increased by 20 degrees, this time is 1718 hours, which is shortened by 112 hours; Reduce the temperature regime by 20 e, with a time value of 2016 h, and extend it by 116 h; The annealing heating time under the highest temperature regime is shortened by 218 hours compared to the lowest temperature regime.
5. Conclusion
(1) The annealing heating time of steel coils is inversely proportional to the radial equivalent thermal conductivity. When the radial equivalent thermal conductivity increases from 414W/(me) to 1214W/(me), the annealing heating time decreases from 2219h to 1916h, shortening by 313h.
(2) The annealing heating time of steel coil is directly proportional to the outer diameter and height of the steel coil. When the outer diameter of the steel coil increases from 116m to 214m, the annealing heating time of the steel coil increases from 1718h to 2117h, an extension of 319h. When the height of the steel coil increased from 019m to 11Sm, the annealing heating time increased from 1513h to 2112h, an extension of 519h.
(3) The annealing heating time of steel coils is inversely proportional to the circulating gas flow rate and temperature. The circulating gas flow rate increased from 66670m3/h to 72670m3/h, an increase of 6000m3/h. The annealing heating time decreased from 1814h to 1719h, a reduction of 015h. The circulating gas temperature increased from 690 e to 730 e, an increase of 40 e, and the annealing heating time decreased from 2016 h to 171 8h, a reduction of 21.8 h.
(4) Based on the above conclusions, the parameters of Baosteel's online control model were modified to improve the accuracy of the control model in calculating annealing heating time. This not only significantly reduces human intervention in the control process during production, but also improves product quality