RU2280701C1 - Method of annealing cold-rolled strips - Google Patents

Abstract

FIELD: metallurgy; heat treatment of materials.

SUBSTANCE: proposed method consists in heating the roll at temperature interval of 200-570°C at rate of 80-90°C/h followed by heating to temperature of 640-660°C at rate of 20-30°C and heating at rate of 20-25°C/h to annealing point. Besides that, holding at annealing temperature is set according to the following relationship: τ=(0.8-0.9)M, where τ is holding time, h; M is mass of lower roll in stack, t.

EFFECT: reduced duration of annealing at high mechanical properties and improved quality of cold-strip surfaces.

2 cl, 3 tbl, 1 ex

Description

The invention relates to the field of metallurgy, specifically to heat treatment, and can be used for recrystallization annealing of cold-rolled strips of low-carbon steels, wound into coils, in a hydrogen protective atmosphere.

Aluminum-stabilized cold-rolled low-carbon steel used for the manufacture of front parts of passenger car bodies should have a high complex of mechanical properties (Table 1) and a defect-free surface.

Table 1. Mechanical properties of cold-rolled steel 08Y grade σ _in , MPa σ _t , MPa δ ₁₀ % R n 250-300 no more than 170 not less than 46 not less than 1.6 not less than 0.2 Notes: R is the coefficient of normal plastic anisotropy;
n is an indicator of strain hardening.

To ensure a given set of mechanical properties, rolls of cold-rolled strips are subjected to recrystallization annealing in a protective atmosphere in single-foot bell-type furnaces with gas heating.

A known method of heat treatment of coils of cold rolled strips of mild steel, including heating installed in the stack of rolls at a speed of 38-55 ° C / h to a temperature of 520-550 ° C, holding at this temperature for 18-22 hours, reheating at a speed of 15 -30 ° C / h to the annealing temperature, holding at the annealing temperature for 20-24 hours, delayed cooling to a temperature of 420-430 ° C and the final cooling of the rolls at an arbitrary speed [1].

The disadvantages of this method are that the annealing has a long duration and does not provide the required mechanical properties of the cold-rolled strips.

There is also known a method of heat treatment of cold rolled sheet metal, including heating a stack of rolls to a temperature of 200-400 ° C, holding for 4-6 hours, reheating to a temperature of 620 ° C, holding for 8-10 hours and final heating to annealing temperature 690-710 ° C with exposure for 28 hours [2].

Annealing by this technology has a long duration and does not provide the required mechanical properties of cold-rolled strips.

The closest in technical essence and the achieved results to the present invention is a method of recrystallization annealing of cold rolled mild steel strips for deep drawing, stabilized by aluminum, wound into rolls weighing 20-30 tons. The method includes heating the rolls stacked on top of one another to intermediate temperatures 580 -600 ° C and 640-660 ° C with holdings at these temperatures and the final heating to an annealing temperature of 710-730 ° C with holding at the annealing temperature [3].

The disadvantages of this method are that holding at intermediate temperatures lengthens the duration of annealing. In addition, the finished cold-rolled metal has low mechanical properties, as well as surface defects due to the welding of coils of coils.

The technical problem solved by the invention is to reduce the duration of annealing while ensuring high mechanical properties and surface quality of cold-rolled strips.

To solve the technical problem in the known method of annealing cold rolled strips for deep drawing, wound into rolls, including heating installed on top of each other in the pile of rolls to annealing temperature of 710-730 ° C and regulated exposure at this temperature, according to the proposal of the rolls in the temperature range of 200 -570 ° C is heated at a speed of 80-90 ° C / h, then heated to a temperature of 640-660 ° C at a speed of 20-30 ° C / h, after which heating is carried out at a speed of 20-25 ° C / h to the annealing temperature . In addition, the shutter speed at the annealing temperature is set by the ratio:

τ = (0.8-0.9) · M,

where τ is the exposure time, h;

M - the mass of the bottom in the foot of the roll, t

The essence of the proposed method is as follows. The heating of the stack of rolls in the temperature range of 200-570 ° C at a speed of 80-90 ° C / h avoids deformation and adhesion of the external and internal coils. Upon subsequent heating at a speed of 20-30 ° C / h to a temperature of 640-660 ° C, the temperature gradient decreases, the inhibiting phase precipitates from the solid solution - aluminum nitrides, which favorably affects the processes of subsequent recrystallization of the deformed microstructure of steel. In the process of heating at a speed of 20-25 ° C / h to an annealing temperature of 710-730 ° C, further equalization of the temperature fields of all cage rolls occurs, the primary recrystallization processes start. The ferrite matrix is purified from carbides. Due to this, during the holding time at the annealing temperature, the collective crystallization processes are completely completed, a uniform microstructure of steel with pancake-shaped grains is formed. Moreover, since the lower roll in the foot, which has the largest mass (for better stability of the foot and to prevent slipping of the outer turns of the coil installed above), is lagging behind heating, the minimum required holding time at the annealing temperature is determined based on its weight from the proposed empirical ratio.

Heating rates (80-90 ° C / h, 20-30 ° C / h, 20-25 ° C / h), as well as temperature ranges (200-570 ° C, 640-660 ° C) are selected so that to provide high mechanical properties of the finished cold-rolled metal with a minimum heating time and to eliminate the need for aging coils at intermediate temperatures.

The absence of endurance at intermediate temperatures and the exact determination of the duration of cage aging (by weight of the bottom in the roll foot) at the annealing temperature reduces the annealing time while simultaneously obtaining high mechanical properties and surface quality of cold-rolled strips.

It has been experimentally established that an increase in the heating rate of more than 90 ° C / h in the temperature range of 200-570 ° C leads to an increase in the unevenness of heating of the coils, deformation of the external and internal coils due to increased thermal expansion, and a deterioration in the uniformity of mechanical properties along the length of the strips. A decrease in the heating rate of less than 80 ° C / h prolongs the annealing time. In this case, to a temperature below 200 ° C, heating can be carried out with maximum intensity without danger of deterioration of the quality of the metal, and at temperatures above 570 ° C, the heating rate should be reduced, because otherwise, the mechanical properties will deteriorate and the coils of the rolls will be welded.

When heated at a rate below 20 ° C / h to a temperature above 660 ° C, the annealing time will be extended. An increase in the heating rate of more than 30 ° C / h, as well as a decrease in the temperature of its completion below 640 ° C, leads to a deterioration in the complex of mechanical properties of cold-rolled strips.

Heating at a rate of less than 20 ° C / h to an annealing temperature above 730 ° C will lengthen the annealing time, will lead to the welding of coils of rolls. An increase in the heating rate of more than 25 ° C / h leads to the formation of an uneven structure and properties, and a decrease in the temperature of recrystallization annealing of less than 710 ° C degrades the mechanical properties of the cold-rolled strips and requires an extension of the annealing time.

Studies have shown that with an empirical coefficient of proportionality k less than 0.8 due to the incompleteness of the recrystallization process, the mechanical properties of the lower roll in the foot do not correspond to the required values and are uneven along the length of the strip. An increase in the coefficient k of more than 0.9 leads to an irrational lengthening of the annealing time.

Method implementation examples

Cold-rolled strips with a cross section of 0.7 × 1550 mm made of steel grade 08Yu, wound into coils, are subjected to recrystallization annealing. Lower in the foot on the stand of the muffle bell furnace set a roll weighing M = 25 tons. A convector ring is placed on the upper end of this roll, on which a roll of a middle tier weighing 20 tons is installed. A convector ring is also placed on its end and a roll of the upper tier weighing 18 tons is installed. The axial hole of the upper tier roll is covered with a lid. A stack of rolls is covered with a muffle, on top of which a heating cap is installed. The muffle space is filled with hydrogen and the metal cages are heated to a temperature of T ₁ = 200 ° C at the highest possible speed. Then, in the temperature range from T ₁ = 200 ° C to T ₂ = 550 ° C, heating is carried out at a speed of V ₁ = 85 ° C / h. The heating speed and temperature are controlled by a bench thermocouple in contact with the bottom end in the roll foot.

Upon reaching a temperature of T ₂ = 550 ° C, the heating rate is reduced to a value of V ₂ = 25 ° C / h, with which the metal continues to be heated to a temperature of T ₃ = 650 ° C. Upon reaching the metal temperature T ₃ = 650 ° C, the heating rate is again reduced to a value of V ₃ = 22.5 ° C / h. Heating at this speed is carried out to the annealing temperature T ₄ = 720 ° C. The exposure time τ at the annealing temperature is determined by the ratio: τ = k · M = 0.84 · M = 0.84 · 25 = 21 (h). After the exposure time τ, the heating cap is removed and the stack of coils under the muffle is cooled to an unpacking temperature of 90 ° C.

Thanks to the use of the proposed recrystallization annealing modes, its duration before cooling began to decrease and amounted to τ _о = 32.5 hours. At the same time, high mechanical properties were achieved in the absence of defects on the surface of cold-rolled strips.

Table 2 shows the options for implementing the method of annealing cold rolled strips, and in table 3 - indicators of their effectiveness.

Table 2. Modes of recrystallization annealing of strips made of 08Yu steel No. p / p T ₁ , ° C V ₁ , ° C / h T ₂ ° C V ₂ ° C / h T ₃ ° C V ₃ , ° C / h T ₄ ° C k, h / t one. 190 78 560 eighteen 630 eighteen 700 0.7 2. 200 80 570 twenty 640 twenty 710 0.8 3. 200 85 570 25 650 22.5 720 0.84 four. 200 90 570 thirty 660 25 730 0.9 5. 210 95 580 32 670 26 740 1,0 6. twenty fifty 590 40 650 eleven 720 - Table 3. Strip quality indicators and annealing period No. p / p σ _in , MPa σ _t , MPa δ ₁₀ % R n Surface defects τ ₀ , h one. 350 220 32 1,2 0.1 presence. thirty 2. 300 170 46 1,6 0.2 absent. 31.6 3. 275 160 52 1.8 0.4 absent. 32,5 four. 250 165 fifty 1.7 0.3 absent. 32,4 5. 230 180 44 1,2 0.1 presence. 34,4 6. 300 175 45 1,1 0.1 presence. 47

From the data presented in tables 2 and 3 it follows that when implementing the proposed method (options No. 2-4), annealing time is reduced while ensuring high mechanical properties and surface quality of cold-rolled strips in comparison with the prototype method (option No. 6) . With exorbitant values of the declared parameters, in one case, there is a deterioration in the mechanical properties and surface quality of the strips (option No. 1), and in the other case, when the mechanical properties and quality of the strips deteriorate, the annealing time increases (option No. 5).

The technical and economic advantages of the proposed method consist in the fact that during its implementation the heating is carried out without holding at intermediate temperatures with extreme heating rates in various temperature ranges, excluding the welding of coil turns and ensuring the formation of the optimal microstructure and mechanical properties of mild cold-rolled steel. In addition, the exposure time at the annealing temperature is set depending on the mass of the lower foot roll lagging behind heating. Due to this, the exposure time is minimized at an annealing temperature of 710-730 ° C.

Using the proposed method provides an increase in the profitability of the production of cold-rolled strips from low-carbon steels by 10-15%.

Information sources

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

2. Auth. St. USSR No. 1461771, IPC C 21 D 9/46, 1989

3. Auth. St. USSR No. 1337425, IPC C 21 D 9/48, 1987 - prototype.

Claims (2)

1. The method of annealing cold rolled strips for deep drawing, wound into rolls, comprising heating installed on top of each other in the stack of rolls to an annealing temperature of 710-730 ° C and regulated shutter speed at this temperature, characterized in that the rolls in the temperature range of 200-570 ° C is heated at a rate of 80-90 ° C / h, then heated to a temperature of 640-660 ° C at a speed of 20-30 ° C / h, after which heating is carried out at a speed of 20-25 ° C / h to the annealing temperature. 2. The method according to claim 1, characterized in that the shutter speed at the annealing temperature is set by the ratio τ = (0.8-0.9) · M, where τ is the exposure time, h; M - the mass of the bottom in the foot of the roll, t