RU2294388C1 - Method of annealing of the cold-rolled strips - Google Patents

Abstract

FIELD: metallurgy industry; methods of annealing of the cold-rolled strips.

SUBSTANCE: the invention is pertaining to the field of metallurgy industry, in particular, to he thermal treatment and may be used at the recrystallization annealing of the tightly wound rolls of the cold-rolled strips from made out of the low-carbon steel in the single-stack bell-type furnaces with the hydrogen protective atmosphere. The problem of the invention consists in reduction of the annealing duration, power inputs and in the increased productivity of the production process. For this purpose the heating duration from the temperature of 190-210°C up to the annealing temperature of 670-710°C and aging at the annealing temperature is set depending on the mass of the lower roll in the stack according to the ratios: τ_l = (0.43 - 0.50)·M, τ_up = (0.44 - 0.52·M, where τ_l and τ_up - duration of the heating and aging accordingly, hours; M is the mass of the lower roll. The method ensures the increase of the profitability of the annealing of the low-carbon steel strips by 12-20 %.

EFFECT: the invention ensures the increased profitability of the annealing of the low-carbon steel strips by 12-20 %.

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Description

The invention relates to metallurgy, specifically to heat treatment, and can be used for recrystallization annealing of tightly wound rolls of cold-rolled strips of mild steel in single-dome bell furnaces with a hydrogen protective atmosphere.

A known method of heat treatment of cold rolled strips of mild steel, wound into coils, in a single-foot bell furnace, comprising heating at a speed of 38-55 ° C / h to an intermediate temperature of 520-550 ° C, holding for 18-22 hours, reheating to annealing temperatures, holding for 20-24 h and cooling [1].

The disadvantages of this method are that the annealing cycle has a long duration and requires large energy costs.

There is also a method of annealing a stack of cold-rolled strips in rolls, including heating the coils to the annealing temperature, holding and cooling, by which the degree of compression of the strip of each coil is determined during cold rolling, and the installation of coils in the stack from bottom to top is carried out with decreasing compression ratio [2].

The disadvantage of this method also lies in the fact that the annealing process takes a lot of time and requires large energy inputs.

The closest in technical essence and the achieved results to the proposed invention is a method of annealing cold rolled strips of mild steel in a single-foot gas furnace with a protective atmosphere, including heating a stack of stacked rolls to annealing temperature of 670-710 ° C, holding and subsequent cooling [3].

The disadvantage of this method of annealing is as follows. Cold rolled coils of various weights are annealed. The total time of heating and holding the stack of rolls at the annealing temperature during which the energy of the burned gas is consumed is set constant regardless of the mass of the lower roll in the stack, which is lagging behind heating; therefore, when annealing a lightweight cage, it exceeds what is actually necessary. The consequence of this lengthening of the recrystallization annealing cycle is the low productivity of the process and the excessive consumption of fuel.

The technical problem solved by the invention is to reduce the duration of annealing and energy consumption, increasing the productivity of the process.

To solve the technical problem in the known method of annealing cold rolled strips of mild steel in a one-stop cage furnace, including heating a stack of stacked rolls to annealing temperature of 670-710 ° C, holding and subsequent cooling, according to the proposal, the duration of heating from a temperature of 190- 210 ° C to the annealing temperature and holding at the annealing temperature are set depending on the mass of the bottom in the roll foot according to the ratios:

τ _n = (0.43-0.50) · M,

τ _in = (0.44-0.52) · M,

where τ _n , τ _in - the duration of heating and exposure, respectively, h;

M is the mass of the lower roll, t

The invention consists in the following. High plastic properties of the cold-rolled strip of mild steel are achieved as a result of recrystallization annealing at a temperature of T ₀ = 670-710 ° C. During annealing of coils in a single-stage bell furnace, the duration of heating and aging should ensure full heating over the cross section of all coils of coils to the annealing temperature, which ensures a given level of mechanical properties, stable along the length of the strips. Since rolls of different weights arrive at the recrystallization annealing, in the existing methods the heating and holding times are set constant, based on the largest possible mass of the cage, rightly assuming that with lower masses of the cage, full heating over the cross section of all the bales will be achieved all the more.

Studies have shown that the bottom roll is the most lagging behind in heating in a single-foot cage oven. The matter is compounded by the fact that for greater stability of the foot, the lower mass is installed on the bottom of the roll. Therefore, it was suggested that the necessary heating and holding times of the entire cage be determined from the condition of uniform heating to the annealing temperature of the lower roll, because while the rolls of the upper tiers are guaranteed to be heated to the annealing temperature. Also, according to the results of measuring the temperature field of the lower foot roll, it was found that the heating time required for its complete and uniform heating from temperature T _n = 190-210 ° C should be directly proportional to its mass with a proportionality coefficient k _n = 0.43-0, 50, and the exposure time is also directly proportional to its mass, but with a proportionality coefficient k _in = 0.44-0.52. It is noted that for these values of k _n, k, and T _{o to} be achieved at the same time the smallest possible cages for the entire time of heating and aging, while ensuring predetermined mechanical properties of cold-rolled steel strips of maloutlerodistoy due to completion of crystallization processes of the deformed microstructure.

A decrease in the annealing temperature of less than 670 ° C degrades the exhaust properties of mild steel: the tensile strength becomes higher than 390 N / mm ² and the relative elongation is lower than 28%, which is unacceptable. An increase in the annealing temperature of more than 710 ° C increases energy consumption and leads to the welding of coil turns.

The temperature of heating onset T _{n is} limited to 190-210 ° C due to the fact that at lower temperatures of the lower coil, heating of the cage can be carried out with the maximum intensity possible under the conditions of the heat output of the furnace for the shortest possible time, without the risk of welding coils and thermal deformation of the coils . At a temperature of T _n below 190 ° C by the time the heating and soaking is complete, the temperature field of the lower roll will not be completely equalized, the plastic properties of the strip will deteriorate, which is unacceptable. An increase in temperature T _n above 210 ° C will lead to an increase in the total heating time and energy consumption, a decrease in the productivity of the annealing process.

When the proportionality coefficient k _{n is} less than 0.43 and k _is less than 0.44 due to an increase in the heating intensity, deformation and welding of the coils of the upper tier rolls occur, the mechanical properties of the annealed strips of mild steel deteriorate, which is unacceptable. With an increase in k _{n of} more than 0.50 and k _in more than 0.52, there is an extension of the heating time, an increase in energy consumption, and a decrease in the productivity of the recrystallization annealing process.

An example implementation of the method

Cold-rolled strips with a cross section of 0.7 × 1450 mm made of low-carbon steel grade 08 ps, wound in dense coils, are assigned to recrystallization annealing. Strips after annealing should have the following mechanical properties (GOST 9045): σ _in = 250-390 N / mm ² ; δ ₁₀ ≥28%. To conduct annealing, a roll weighing M = 23 t, which is the bottom in the foot, is installed on the stand of a single-stage bell furnace using an electric bridge crane. A convector ring is laid on the upper end of the roll and a second tier roll with a lower mass of 20 tons is installed. A third tier roll of 18 t mass is also installed on top of the second tier roll through the convector ring.

The formed foot is closed with a muffle, the joint of the muffle with the stand is sealed, and the muffle space is filled with hydrogen.

Then, a heating cap is installed on the stand, gas burners are lit, and the cages are heated up to the maximum possible intensity to a temperature T _n = 200 ° C. The heating temperature is controlled by a bench thermocouple in contact with the end of the lower roll.

The heating of the lower roll from the temperature T _n = 200 ° C to the annealing temperature T ₀ = 690 ° C is carried out for a time τ _n , comprising:

τ _n = k _n · M = 0.46 · 23 = 10.58 (h),

those. with an average heating rate of 46.3 ° C / h, which is set using the programmer.

At the annealing temperature, the roll cage is held for a time τ _in , which is

τ _in = k _in · M = 0.48 · 23 = 11.04 (h).

After completion of exposure at the annealing temperature, the heating cap is turned off, removed from the stand and the stack of coils under the muffle is cooled to an unpacking temperature of 90 ° C.

Due to the fact that the heating time τ _n from the temperature T _n = 200 ° С to the annealing temperature Т _о = 690 ° С and the holding time τ _{в are} determined based on the mass M of the lower roll according to the proposed ratio, heating and aging are carried out for the minimum possible time τ _n + τ _in = 10.58 h + 11.04 h = 21.62 h. At the same time, all cage rolls have a uniform cross-section temperature at the end of exposure, and also acquire high exhaust properties, uniform along the length of the strips.

Minimization of τ _n and τ _in provides a reduction in the duration of annealing and energy consumption to the value of Q = 0.47 GJ / t, increasing the productivity of the process to P = 2.0 t / h per bench while maintaining a given level of mechanical properties: σ _in = 320 N / mm ² ; δ ₁₀ = 36%.

Implementation options of the proposed method and indicators of their effectiveness are given in the table. From the table it follows that in the case of the implementation of the proposed method (options No. 2-4, 7-9, 12-14), a reduction in the duration of annealing and energy consumption, an increase in the productivity of the process with different mass of the lower roll and cage are achieved. At the same time, the required level of mechanical properties of the annealed metal is achieved.

With exorbitant values of the declared parameters (options No. 1, 5, 8, 10, 11, 15), the required mechanical properties of the metal are not provided, the annealing time and energy consumption increase, the process productivity decreases. In the case of using the prototype method (option 16), although the required mechanical properties of the metal are achieved, there is also a longer annealing time and energy consumption, low process productivity.

The technical and economic advantages of the proposed method are that the determination of the duration of heating and exposure depending on the mass of the lower foot roll lagging behind in heating according to the proposed ratios, at an annealing temperature of 670-710 ° C, minimizes the heating and holding time of the charge at the annealing temperature, reduces energy consumption and increases the productivity of the process while maintaining the required level of mechanical properties.

As a basic object in determining the economic efficiency of the proposed technical solution adopted the prototype method. The application of the proposed method provides an increase in the profitability of annealing strips of mild steel by 12-20%.

Literary sources used in the preparation of the description of the invention:

1. Auth. testimonial. USSR No. 1659500, IPC C 21 D 9/46, 1991

2. Auth. testimonial. USSR No. 1344796, IPC C 21 D 9/663, 1987

3. SS Guseva and others. Continuous heat treatment of steel sheet. M., Metallurgy, 1979, S. 18-25 - prototype.

Table Annealing modes of cold-rolled strips of 08 kp steel and indicators of their effectiveness No. p / p T _o , ° C M, t k _n k _in τ _n , h τ _in , h τ _n + τ _in , h T _o , ° C Q, GJ / t R, t / (h · stand) Mechanical properties σ _in , MPa δ ₁₀ % one 180 fifteen 0.42 0.43 6.30 6.45 12.75 660 0.45 2.0 410 26 2 190 fifteen 0.43 0.44 6.45 6.60 13.05 670 0.49 2.1 385 29th 3 200 fifteen 0.47 0.47 7.05 7.05 14.10 700 0.50 2.0 300 35 four 210 fifteen 0.50 0.52 7.50 7.80 15.30 710 0.51 1.9 250 38 5 220 fifteen 0.51 0.53 7.65 7.95 15.60 720 0.61 1.4 240 27 6 180 23 0.42 0.43 9.66 9.89 19.55 660 0.45 1.9 420 24 7 190 23 0.43 0.44 9.89 10.12 20.01 670 0.46 2.1 380 35 8 200 23 0.46 0.48 10.58 11.04 21.62 690 0.47 2.0 320 36 9 210 23 0.50 0.52 11.50 11.96 23.46 710 0.48 1.9 260 38 10 220 23 0.51 0.53 11.73 12.19 23.92 715 0.61 1,5 230 27 eleven 180 thirty 0.42 0.43 12.60 12.90 25,50 665 0.45 2,5 430 22 12 190 thirty 0.43 0.44 12.90 13,20 26.10 670 0.43 2.2 380 thirty 13 200 thirty 0.48 0.46 14.40 13.80 28,20 680 0.45 2.2 320 36 fourteen 210 thirty 0.50 0.52 15.00 15.60 30,60 710 0.47 1.9 255 37 fifteen 220 thirty 0.51 0.53 15.30 15.90 31,20 720 0.61 1,6 240 28 16 - fifteen - - 12.00 36.00 48.00 690 1.34 0.6 260 32

Claims (1)

The method of annealing cold rolled strips, including heating a stack of stacked rolls to an annealing temperature of 670-710 ° C, holding and subsequent cooling, characterized in that the duration of heating from a temperature of 190-210 ° C to the annealing temperature and the exposure time at the annealing temperature are set depending on the mass of the bottom in the foot of the roll according to the ratios τ _n = (0.43-0.50) · M; τ _in = (0.44-0.52) · M, where τ _n , τ _in - the duration of heating and exposure, respectively, h; M is the mass of the lower roll, t