SU1766986A1 - Method of cold-rolled strip steel production - Google Patents

Abstract

SUMMARY OF THE INVENTION: in a method including cold rolling of stripes with a roughness value behind the last mill stand 0.4 ... 0.2 Ra, two-stage annealing of tightly wound coils in bell-type furnaces and training, in order to improve the surface quality by reducing the fracture defect and saving fuel, heating coils in the first stage of annealing is carried out with fuel consumption in the amount of 50 .... 70% of the maximum fuel consumption per kiln. 2 tab. 2 Il.

Description

The invention relates to ferrous metallurgy, in particular, to the production of sheet metal, and improves the process of annealing cold-rolled steel, mainly of the type 08U, in densely wound rolls in bell-type single-stop furnaces.

Known methods for the production of cold-rolled strip steel, including cold rolling of roughness strips behind the last mill stand in the range of 0.4 ... 2.0 Ra, two-stage annealing of tightly wound coils in bell-type furnaces at 540 ... 560 and 660 ... 700 ° With and training.

The main disadvantages of the known methods are the sticking and welding of the coils in the annealing process, the high consumption of equivalent fuel.

A known method for the production of cold-rolled strip steel, selected as a prototype, is carried out as follows. Refined hot rolled coils are fed to a continuous multi-mill mill where they cold-roll the strips with a final thickness of 0.35-2.0 mm and wind them into tightly wound coils. The deadness of the work rolls of the last stand of the continuous cold rolling mill is such that when the strip is rolled up into a roll, its roughness may be 0.4-2.0 Ra. After rolling, the density rolls are fed to the thermal compartment for annealing. At the stand of the stack of the pack oven, a stack of coils is formed, it is heated with a muffle, protective gas is fed into the baffle space, a circulation fan is turned on, and a heating hood is installed. The furnace is heated by burning gaseous fuel using atmospheric injection or two-wire burners in the annular gap between the muffle and the refractory masonry of the heating cap. The fuel is combusted at the bottom of the heating cap. The products of combustion, moving upwards through the annular gap, heat the muffle and are removed into the smoke hogs. The coils are heated by the transfer of heat by radiation from mufses.

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l on the outer surfaces of the rolls and the circulation of protective gas in the underfuffle space. The protective gas absorbs heat from the muffle and, when circulating through the vector rings, transfers heat to the ends of the rolls and partially to the outer and inner surfaces of the foot. In the process of heating, the maximum amount of fuel is supplied to the furnace until the control cap in the working space of the heating cap, for example, 830-850 ° C. After that, the fuel consumption for combustion is reduced by 20–25% of the maximum value, and the rolls of heels are heated at this fuel consumption until the first holding temperature in the under-riffle space of 540–560 ° C. In the process of holding, fuel consumption is maintained at a level that ensures the temperature of 540–560 ° C, while the temperature of the heating cap is reduced to 700–750 ° C. At the end you; The holders at the first stage again supply the maximum amount of fuel to the furnace until the temperature in the working space, for example, 830-850 ° C, and is heated until the second holding temperature in the under-riffle space is 660-700 ° C. Regulation of fuel consumption is similar to regulation for heating metal at the first exposure. After the end of holding at 660-700 ° C, the heating cap is removed, the rolls are cooled under the muffle to the unpacking temperature, for example 100-140 ° C, and transferred to the forced cooling or natural cooling section to a final temperature of 40-60 ° C. Cooled to 40-60 ° C, the rolls are trained with a total reduction ratio of 0.5-3.0% and are shipped to consumers,

The main disadvantage of this method is the low surface quality of cold rolled strips and the high consumption of reference fuel for their production. The low quality of the surface is caused by sticking and welding of coils in the annealing process. When unwinding the coil during the process of training, fracture lines appear on the surface of the strip, which deteriorate the presentation of the product and serve as a defect for converting the sheet to the lower surface finish group. One of the main causes of sticking and welding of coils during annealing is a large temperature difference across the cross section of annealed coils, reaching 140-200 ° C. Temperature differential caused by the following reasons. Heating of the coils in the furnace occurs in the radial and axial directions, i.e. through lateral and internal surfaces and through end surfaces. In the axial direction, the coefficient of thermal conductivity is equal to the coefficient of thermal conductivity of the metal, whereas in the radial direction, due to the poor contact between the turns, it is an order of magnitude lower. As a result, the coils of the central thickness of the winding are most slowly heated. In addition, the transfer of heat by radiation, directly, from the muffle to the outer side surface of the rolls leads to overheating of the outer turns, because The intensity of heat supply to the side surface is significantly higher than the intensity of heat transfer in the radial direction. The temperature difference across the cross section leads to the appearance of radial thermal stresses in rolls, reaching values of 40-45 N / mm2 in the coil-thickened cores. By the end of heating, as the temperature drop decreases, thermal stresses decrease to 8-10 N / mm2. In the process of heating, thermal expansion of the coils in diameter occurs under axial load. As a result, the radial thermal stresses at maximum temperature differences increase the magnitude of the contact compressive stresses between turns, caused by the tension force during the winding of the rolls, by 5–5.5 N / mm2. The magnitude of the contact stresses of compression of the central part of the coil winding with a tension of 30 N / mm2 is equal to 5-6 N / mm2; 50 N / mm2 - 9-10.5 N / mm2, 70 N / mm2 - 11-13 N / mm2; 100 N / mm 2-15-18 N / mm 2. Experimental data show that when the magnitude of contact stresses at a level of 11–18 N / mm, coils of coils adhere, resulting in the appearance of kink lines in the strip. Therefore, even with minimal tensions during coiling, the magnitude of the contact stresses due to their summation with the radial thermal stresses exceeds in certain cases the limiting values, above which the sticking of the coils is observed.

Another disadvantage of the known method is the high consumption of equivalent fuel for heat treatment of coils. Conditional fuel consumption during annealing of cold-rolled non-aging steel of type 08U reaches 55-60 kg. t / t metal. The high consumption of reference fuel is due to the fact that in order to reduce the temperature difference over the cross section of coils from 140-200 to 20-30 ° C, it is necessary to conduct a long exposure to 20-30 hours.

The aim of the invention is to improve the surface quality by reducing the fracture defect and saving fuel.

To do this, in the production of strips of non-aging steel grades, the coils in the first annealing stage are heated with fuel consumption in an amount of 50 ... 70% of the maximum fuel consumption per kiln.

The production method of cold-rolled strip steel includes cold rolling of roughness strips behind the last mill stand in the range of 0.4-2.0 Ra, two-stage annealing of tightly wound rolls in bell-type furnaces 540-560 and 660-700 ° С, respectively, and training.

The proposed method differs from the prototype in that in the production of strips of non-aging steel grades, the coil heating in the first annealing stage is carried out with a fuel consumption of 50 ... 70% of the maximum consumption per kiln.

The analysis of the known technical solutions and the proposed similar features are not found. Consequently, the proposed technical solution has significant differences.

The proposed method for producing cold rolled strip steel is carried out as follows.

Cold rolled strips with a thickness of 0.35-2.0 mm are made of hot rolled coils in the cold rolling mill, rolled up into coiled windings and fed to the thermal compartment. The roughness of the bands behind the last stand of the mill, depending on the purpose, is 0.4-2.0 Ra. At the stand of the bell-type furnace, a stack of rolls is formed, covered with a muffle and a heating cap is installed. In the process of heating until the control temperature reaches in the working space, for example, 830-850 ° C and until the temperature of the first annealing stage reaches 540-560 ° C in the underfloor space, the fuel consumption is set to 50 ... 70% of the maximum fuel consumption on the stove. The desired flow rate is established, for example, by appropriately opening the valve on the gas supply pipe to the furnace, or by disconnecting a number of burners. At the end of the first annealing stage, the temperature rise in the working space of the furnace and in the undermuffle space in the second annealing stage is carried out at maximum fuel supply to the burners.

Annealed coils, cooled in special areas to 40-60 ° C and subjected to training.

Improve surface quality in the production of strip of non-aging steel grades using the proposed

the method is achieved by heating the coils in the first annealing stage with fuel consumption equal to 50-70% of the maximum consumption. Reducing the fuel supply to the stove reduces the rate of temperature rise in the stove. As a result, the heating rate of the muffle decreases and, accordingly, the heat flux from the muffle to the outer lateral surface of the rolls decreases, its value is at the same level as

the amount of heat flux in the radial direction through the thickness of the winding coil. Convective heat transfer is almost unchanged, since is determined mainly by the velocity of the protective gas in

annular gap between muffle and foot rolls. Consequently, the heating rate of the outer coils decreases at a constant heating rate, the central coils lagging in heating, i.e.

The temperature difference over the cross section of the coils during the annealing process is lower than during annealing with the maximum fuel consumption. Increasing the uniformity of heating over the cross section of the rolls leads to a decrease in radial

thermal stresses. The total amount of contact and radial stresses acting on the coils of the coils decreases accordingly. The likelihood of sticking together and welding turns decreases

rolls, i.e. reduced defect kink on the surface of the annealed strips. The optimal fuel consumption in the first stage of annealing was found experimentally. At the same time, it was assumed that with optimal fuel consumption, the temperature difference across the cross section of the coils during the heating process should be minimal. At the same time, the heating temperature of the metal at the lagging point to the end of the total duration of the first annealing stage should not be lower than the temperature of the metal during heating with maximum fuel consumption. In tab. 1 shows the experimental data on the heating of the stack of coils with different fuel consumption in the first stage of annealing.

The results of experimental studies show that the most optimal fuel consumption in the first stage of annealing is the flow rate equal to

50-70% of maximum consumption. The temperature difference over the cross section of coils in this range does not exceed 85-100 ° C, i.e. decreases 1.5-2 times; by the end of heating it does not exceed 40-45 ° C, radial thermal stresses also decrease 1.52 times. The duration of exposure at

preserving the total time of the first annealing stage by reducing the rate of temperature rise in the undermuffed space and increasing the uniformity of heating is reduced by 9-15 hours. But the metal temperature at the lagging point, which ultimately determines the mechanical properties, corresponds to the metal temperature during annealing with maximum fuel consumption . With an increase in fuel consumption of more than 70% of the maximum, the heating irregularity over the cross section of coils increases, the radial thermal stresses and the equivalent fuel consumption increase. By reducing the fuel consumption to less than 50% of the maximum consumption, the temperature parameters of heating over the cross section of the coils are improved. However, the metal is not heated to the required temperature of 495-500 ° C, it is necessary to increase the holding time until this temperature is reached, which leads to a decrease in the productivity of the bell furnace.

The reduction in the consumption of reference fuel using the proposed method is achieved by reducing by 30-50% the fuel consumption for combustion. According to the table. 1, the fuel consumption in the first annealing stage is 16-22 kg. Ton / ton. If we consider that the fuel consumption at the second stage of annealing is 27-30 kg.w.t / t, then using the proposed method will lead to a reduction in fuel by an average of 15-17%.

The proposed method for the production of cold-rolled strip steel was tested under industrial conditions NL MK. For this purpose, 6 rolls weighing 28.5 tons each for the final thickness of 0.5 mm cold-rolled strips were rolled to the 2030 mill from hot-rolled strips with a thickness of 2 mm and a width of 1360 mm. In the last stand of the mill, grinding work rolls were installed, which ensured the roughness of the strips during winding into rolls equal to 0.45 Ra. The tension during coiling was 2.5 kg / mm. From laminated rolls in the thermal compartment formed two cages. One foot of the rolls was annealed according to the mode of the current technological instruction. The second stack of coils was annealed in compliance with only the total time of the first stage, and at the second stage of annealing, the total time and duration of exposure corresponded to the parameters of the process instruction. Coke-domain gas with a calorific value of 1600 kcal / m3 was used as fuel. When annealing by a known method at the first stage of annealing, the maximum gas flow rate was 670 m3 / m, while annealing by the proposed method was 350 m3 / h.

FIG. Figures 1 and 2 show the thermal and temperature conditions of annealing the coil foot, according to the known and proposed methods.

From the data obtained it follows that with

using the proposed method, the maximum temperature difference over the cross section of coils in comparison with the known method decreased in the first stage annealing from 150 to 95 ° С, i.e. by 37%, at the second stage of annealing - from 90 to 75 ° C. The reduction in fuel consumption in the first stage also led to an increase in the uniformity of heating along the height of the foot. Temperature difference across

5, in the process of heating as a whole, the foot height decreased by 40 ° C, the duration of exposure at the first annealing stage was reduced, as compared with the technological instruction mode, by 14 h. The metal temperature in

0 lagging points by the end of the first exposure reached 500 ° C. In tab. 2 shows the results of the annealing of the foot rolls on the known and proposed methods.

Data analysis table. 2 shows that

The use of the proposed method made it possible to reduce fuel consumption from 57.2 to 48.3 kg.w.t / t, i.e. by 15%. The mechanical properties of the annealed sheet correspond to the properties of the metal annealed according to the well-known method. The sorting data of the prodressed sheets showed that an increase in the coil heating uniformity over the cross section and a corresponding decrease in the radial thermal stresses

5 led to an improvement in the surface quality of the sheets by reducing the fracture defect by 2.6%.

Thus, the use of the proposed method for the production of cold-rolled strip steel in comparison with the prototype makes it possible to improve the quality of the strip surface by reducing the fracture defect by 2.6% and reduce fuel consumption by an average of 15%.

Claims (1)

5 claims Method for the production of cold-rolled strip steel, predominantly non-aging, including cold rolling of strips with a roughness value at the last mill stand in the range 0.4 ... 2.0 Ra, two-stage annealing of tightly wound coils in bell-type furnaces at 540 ... 560 and 560 ... 700 ° C, respectively, and training, characterized in that, 5 in order to improve the surface quality by reducing the fracture defect and saving fuel, heating the coils in the first annealing stage is carried out with fuel consumption in an amount of 50 ... 70% of the maximum fuel consumption per kiln. T a b l and c  Numerator - maximum temperature difference; the denominator is the minimum temperature difference. 10 20 3D W 50 60 70 BO 90 fOO YAO f2O 13D T, g 1 table 2 S 20 JO 40 SO 60 70 80 90 WO ffu fZO ffl T, r FIG. Z