Selection of intermediate blank insulation method for 2250 stainless steel hot continuous rolling production line

Lu question discusses the choice of Li Xiaowu (Taigang 2250 hot rolling project) for the middle billet insulation method of 2250 stainless steel hot strip rolling production line. The insulation method of the intermediate billet becomes the key link of the whole production line. It is adopted for the current hot rolling non-cast steel production line. Comparing the four insulation methods, it is considered that a hot coil box with end insulation boards is also preferred for the side heaters.

Taigang's new 2250 hot continuous rolling production line has an annual production capacity of 4 million tons. Stainless steel 2 million tons will become the world's largest hot rolling production line. Since the deformation resistance of stainless steel during hot rolling is large, it is about 1.2 times that of low carbon steel, and the deformation resistance increases sharply with the decrease of temperature. At the same time, due to the thinner thickness of the intermediate billet during the production of stainless steel, the temperature drop is very fast. When the thinner stainless steel is produced in the 1549 hot rolling line of Taigang, even if the Ericopanel heat shield is used and the speed-up rolling method is adopted, the strip enters the fine The temperature difference between the head and the end of the rolling mill has reached about 100C. The increase of the rolling pressure of the F0 frame is above 500t, and the temperature drop of the tail which can reach 3700t is not only very good for rolling stability, thickness precision, shape control, etc. The difficulty is too great, and the excessive rolling pressure is close to the ultimate load capacity of the equipment, posing a threat to equipment safety. The use of speed-up rolling requires a large surplus of main engine power. In the production, the rolling mill prematurely alarms and stops accelerating, which is more unfavorable for the tail rolling. On the other hand, the too low rolling temperature of the tail also causes edge cracking in the tail of the stainless steel product, which reduces the yield and causes difficulties for subsequent cold rolling. Therefore, the insulation method of the intermediate strip of the traditional hot strip mill is directly related to the quality of the product, the thinning of the product and the ability of the equipment, especially the new 2250 hot rolling production line of stainless steel as the leading product, the intermediate billet heat preservation method. The choice has become one of the most important aspects of the entire line design and plays a vital role in the stainless steel production of the 2250 hot rolling mill.

1 Insulation method of the intermediate billet of the traditional hot strip rolling stainless steel production line Compared with the CSP production line adopting the roller bottom tunnel heating furnace, the insulation method of the intermediate billet of the conventional hot strip rolling stainless steel production line mainly has the following four modes.

Encopanel insulation cover is Encomechi's patented technology, which can reduce the temperature loss of intermediate strips by more than 30C. The current production plants include Taiwan Sinosteel 1 Taigang 1549 hot rolling mill.

Encopanel insulation cover or ordinary passive insulation cover + edge heater uses ordinary insulation cover to keep the insulation effect lower than Enrnpanel insulation cover, and the edge heater gives local heating within about 50mm of the edge of the intermediate blank, usually the temperature range is 30C. ~80C to compensate for the edge temperature is too low, to prevent the stainless steel edge temperature from falling too low plasticity and lead to edge crack defects, in the stainless steel hot rolling production line insulation cover and edge heaters are generally used in pairs. The manufacturers used include Pohang 2 hot rolling, Fuxin 2 hot rolling, Taiwan Sinosteel 2 hot rolling 1.3 active heating type thermal insulation cover, the latest type of thermal insulation cover, Italian Techint technology, namely, General Technology University, 030003, Taigang The upper part of the heat preservation cover of the hot rolling mill is provided with a heating burner to heat the passing intermediate body to increase the overall temperature of the strip. Taking an intermediate blank with a thickness of 30mm as an example, when 12 sets of heat preservation cover are put into place, the temperature drop rate of the intermediate blank can be reduced from 1.55 C/s to 0.41 C/s. The fuel used is natural gas with high calorific value, clean and pollution-free. It has been successfully applied in the Kram factory in Belgium and the Terni plant in Italy, and will become the future development trend.

1.4 The working principle of the hot-rolling hot-rolling box is to carry out the coreless coiling of the intermediate strip rolled out by the roughing mill. After the unwinding, the original strip tail enters the rolling mill first to solve the head-to-tail temperature difference problem.

At the same time, since the strip is tightly wound into a roll, it is difficult to dissipate heat, so the overall temperature drop is greatly reduced.

Another advantage of using the hot coil box is that the intermediate roll length can be greatly shortened, and at the same time, since the temperature of the strip is uniform, the finishing mill can be rolled at a constant speed without increasing the speed, thereby reducing the capacity of the finishing motor. The coreless shaft hot coil box has been successfully applied in the domestic Pangang hot-rolled sheet mill. The measured overall temperature drop of the intermediate billet is about 20C~30C, which reduces the load on the finishing mill, thins the product, increases the weight and The improvement in the quality of the product has played a key role. Stainless steel hot-rolling line with hot-rolling box is hot-rolled (800mm), British Steel, Talbot, Hot Rolling (2030mm), Swedish Svikt Hot Rolling (850mm), Australia BHP West (2050mm), Kawasaki Chiba 2 hot rolling (2050mm) and so on.

The temperature of the intermediate blank entering the F1 frame under the four insulation modes 2 The comparison of the thermal insulation effect of the four insulation methods is 1250mm, the thickness of the intermediate blank is 28mm, and the length of the blank is 70n. The temperature of the intermediate blank into the F1 frame is as shown in the following figure. Show: a) Active heating type insulation cover due to external heat source, the overall average temperature of the table 1 finishing rolling each frame head and tail load finished product specifications, followed by the hot coil box, again Encopanel insulation cover, the overall average temperature is the lowest It is a normal passive insulation cover.

b) From the temperature difference between the head and the tail of the F1 rolling mill, the temperature difference is the hot coil box, the head-to-tail temperature difference is about 20C, followed by the active heating type insulation cover, the head-to-tail temperature difference is about 60C; again the Encopanel insulation cover, the head The temperature difference between the tails is about 80C; the maximum temperature difference between the head and the tail is the ordinary passive heat preservation cover, which is above 90C.

3 Taigang 2250 stainless hot strip mill intermediate blank insulation method selection Active heating type insulation cover is visible, in various insulation sides, active heating type insulation cover insulation effect is the best, is the future development trend, but Taigang due to no Natural gas resources, although the West-East Gas Transmission Project transits through Shanxi, the resources and prices are still unknown, and the risks are too great after the launch. If the fuel is changed to gas, but the gas heat value is low and the site pollution is serious, it can not meet the actual production needs. Therefore, the scheme of adopting the active heating type heat shield can only be abandoned.

3.2 Insulation cover + edge heater scheme and hot coil box selection Taigang's new 2250 hot rolling mill is positioned in the production of wide and thin gauge stainless steel. The design specification of stainless steel is A! SI304 600mm and 4.5mnX2100mm, when using Encopanel insulation cover method, although adopting speed-up rolling, due to the large temperature drop at the head and tail, the rolling load of the front frame increases greatly when rolling to the tail, when rolling ASI304 5mmX1600mm specification When the Encopanel insulation cover is put into use, the difference between the rolling pressure and the head-to-tail rolling pressure of each frame of the finish rolling is shown in Table 1 below: It can be seen from Table 1 that when the Encopanel insulation cover is used to roll 20mmX1250mm and AISI304 stainless steel When F1 and F2 are rolled to the tail, the rolling force increase value ~F4 rolling force is above 3 800t. When rolling 1600mm and AISI304 stainless steel, the rolling force increase value of F1 and F2 rolling to the tail is 1382t and the rolling force of 1169tFi~F4 is above 4100t. If the bending force is also used, the rolling of AGC cylinder is accepted. The force of 4400t is very close to the limit of 4500t. This can not be taken in actual production. Similarly, if the bending force of the bending roller nine F5 has been exceeded, the equipment limit of 4000t has been exceeded. At the same time, under such high load, the finishing of each frame is very difficult to maintain the stability of the rolling and the good shape of the plate. In addition, in the case where the load on each frame is already large, the speed-up rolling is required, and the motor is also operated at full load. After the power alarm occurs and the acceleration is stopped, the rolling process becomes more difficult. Therefore, the use of Encopanel insulation cover method can not guarantee the normal production of stainless steel limit specifications, and further limit the potential of equipment.

If the ordinary passive heat preservation cover is used, because the heat preservation capacity is lower than that of the Encopanel heat preservation cover, the load of the rolling mill at the end of the rolling is increased more, which cannot meet the production of the thin gauge stainless steel.

When the hot coil heat preservation method is adopted, the overall temperature drop of the strip blank is small and the temperature along the length direction is uniform. Therefore, the load change of each frame of the finishing rolling is small, and the rolling force changes of the limit specifications 25n1600nm and AISBm stainless steel are MF1 and F2 respectively. 600t and 5C0t, which is conducive to the stability of rolling. At the same time, constant speed rolling or smaller acceleration speed rolling can be adopted, which reduces the requirement on the capacity of the motor, can fully meet the production of stainless steel limit specifications, and guarantees stable production of stainless steel in a thin gauge size.

In terms of investment and maintenance costs, Encopanel insulation cover + edge heater investment is more than 1.2 times that of hot coil box, and Encopanel insulation cover patented insulation module replacement cost is expensive, side heater power consumption is large, heating coil is also more Expensive, so the former has no advantage in terms of investment costs, operating and maintenance costs.

From the perspective of the temperature drop at the edge, when the hot coil box is used, the residence time of the strip in the middle roller is greater than when the heat shield is used. Therefore, the temperature drop at the edge of the strip is greater than about 20C~40C when using the heat shield, but the heat preservation is used. When the cover is used, the temperature of the tail of the strip is higher than the temperature of the tail when using the heat shield is about 40C~50C. Therefore, when using the hot coil box, the absolute temperature of the tail of the strip is still higher than when the heat shield is used. In the actual production of Taigang 1549 hot rolling mill, the edge cracking of stainless steel occurs mostly in the tail of the strip. The reason is not only related to the temperature drop of the edge, but also related to the absolute temperature of the edge. As long as the edge temperature is higher than the temperature at which the stainless steel is maintained in good plasticity, the edge cracking caused by temperature is eliminated. Foreign countries use hot coil boxes to produce stainless steel production lines. For example, the German Bochum factory with an annual output of more than 1.3 million tons of stainless steel produces no stainless steel edge cracks or rarely occurs. The hot coil box can fully satisfy the mass production of stainless steel. At the same time, with the development of the hot roll box technology, a hot roll box with an end insulation block has been designed to increase the temperature of the edge of the strip by more than 10C~15C.

In order to meet the needs of the 2250 hot rolling mill to develop new stainless steel grades with low plasticity and edge cracking in the future, the front part of the flying shears is reserved with a side heater position, which leaves sufficient room for development in the future.

4 Conclusions By comparing the four intermediate billet insulation methods used in the traditional hot-rolled stainless steel production line, the insulation effect, the edge temperature drop, the finishing mill load, the product thinning and the long-term development are compared. The hot coil box of the heat insulation board and the position of the edge heater at the same time are the better choice for the middle billet insulation method of the new 2250 stainless steel hot strip rolling production line. (Proofreading: Lv Jing)

(Finish)

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