RU2288284C1 - Method of heat treatment of cold-rolled carbon steel - Google Patents

Abstract

FIELD: ferrous metallurgy; heat treatment of rolls of cold-rolled product in bell-type furnace.

SUBSTANCE: proposed method consists in single-stage heating of rolls of cold-rolled sheet in bell-type furnace in shielding atmosphere to 620-660°C, holding the product in furnace during period of time depending on mass of lower layer of charge which is determined by the following formula: δ_hold=14.65+exp(-3.64+0.27M_low.roll-0.00309M² _low.roll), where δ_hold is holding time; M_low.roll is mass of lower roll in charge; then heating is switched off and product is cooled down in furnace to temperature of 550-610°C, after which bell is removed and product is cooled under muffle to temperature not above 140°C. Proposed method makes it possible to reduce sorting-out the product by fracture and sticking defects due to reduced duration of annealing and optimization of holding time.

EFFECT: increased output of furnace; reduced power requirements.

1 tbl, 1 ex

Description

The invention relates to ferrous metallurgy, and in particular to heat treatment in bell furnaces of coils of cold-rolled steel.

A known method of heat treatment of cold rolled steel, including heating in the range from 640 ° C to a temperature of recrystallization annealing at a rate of 5-8 ° C / h and cooling in the range from temperature of recrystallization annealing to 630 ° C at a speed of 2-4 ° C / h ( see SU No. 1183554, C 21 D 9/48, Bull No. 37 of 07.10.85).

The disadvantage of this method is the long duration of annealing, contributing to a loss in productivity, increased energy consumption and increasing the likelihood of the formation of defects "kink" and "spots of adhesion of the weld" at the finished rental.

The closest in technical essence and the achieved results is a method of recrystallization annealing of cold rolled steel coil, by which annealing is carried out in a protective atmosphere at 640-720 ° C, heating to 550 ° C is carried out at an arbitrary speed, and in the range 550-720 ° C at a speed heating not more than 250 ° C / h, after aging, the charge is cooled in a protective atmosphere to 120-180 ° C (see Dedek Vl. Strip steel for deep drawing. M: Metallurgy, 1970, p. 93-117).

The disadvantages of this method are the high temperature of the heat treatment and the unregulated holding time, which leads to increased energy consumption and sorting of finished products according to defects “kink” and “welding sticking spots”, as well as the high temperature of unpacking the coils, which can cause oxidation of the steel surface during cooling on air.

The technical effect when using the invention is to increase the performance of bell-type furnaces and reduce energy consumption while guaranteeing the required set of mechanical properties of sheet metal, as well as to reduce the sorting of steel by defects "kink" and "welding sticking spots".

The indicated technical effect is achieved by heating in a bell furnace of a cage, consisting of one or more coils of cold rolled steel, in a protective atmosphere according to a single-stage mode without limiting the heating rate to a temperature of 620-660 ° C with a minimum exposure in the furnace for a time calculated according to the formula:

τ _vyd = 14.65 + exp [-3.64 + 0.27 · M _{lower steering} - 0.00309 · M ² _nizhn. ),

where τ _vyd - holding time, and _nizhn.rul M - mass of lower roll in the stack, cooled in the furnace under a heating cap with the burner to a temperature of 550-610 ° C, cooled under the muffle and unpacking at a temperature not exceeding 140 ° C.

The essence of the invention lies in the differentiation of the exposure time of the rolls in the bell furnace, depending on the mass of the lower roll in the cage, consisting of one or more rolls. The effect is achieved by optimally determining the exposure time and reducing the total annealing time. In addition, when coils are cooled in the furnace under a heating hood with the burners turned off due to heat transfer from the external, warmer turns to the inside, the temperature is equalized over the entire volume of the coils and the temperature difference in various sections of the coil is eliminated. The cooling of the coils under the heating hood allows the most complete separation of carbon and nitrogen from the ferrite matrix and thus the improvement of the mechanical properties of the rolled products. This eliminates the need for long exposures at elevated annealing temperatures to complete the recrystallization process, improves the productivity of furnaces and reduces energy costs by reducing the total heating and holding time by at least 4-15 hours. In addition, the reduction of the exposure time helps to reduce thermal stresses inside the coil during annealing and cooling and reduces the likelihood of formation of defects "kink" and "spot adhesion welding" on the surface of the annealed strips.

The lower and upper limits of the metal heating temperature and the temperature to which the stack of rolls are cooled under the heating hood are adopted to provide the required values of mechanical characteristics in sheet metal while saving energy. The dependence for determining the exposure time was obtained experimentally, based on the conditions for reaching the temperature of recrystallization throughout the volume of the roll.

The lower limit of the heating temperature of rolled products 620 ° C is determined by the fact that with its further decrease there is an excessive increase in strength properties and a significant decrease in ductility, since the inner coils of the coil may not reach the temperature of completion of the primary recrystallization.

An increase in the heating temperature of rolled products above the upper limit of 660 ° C is impractical, since this leads to a rise in price of steel due to increased energy consumption and can lead to surface defects of strips due to cohesion of coils.

The lower and upper boundaries of the temperature range to which the coils are cooled under a heating cap of 550-610 ° C are selected from the condition of sufficiency for uniform distribution of heat throughout the entire volume of the coil, completion of the recrystallization process in the outer coils of the coils and reduce the likelihood of coalescing due to thermal stresses inside the roll at the initial stage of cooling the charge.

The maximum unpacking temperature of 140 ° C is selected from the condition that there is no oxidation of the metal surface when it is cooled to a training temperature.

The analysis of scientific, technical and patent literature shows the lack of coincidence of the distinguishing features of the proposed method for heat treatment of cold rolled steel with the signs of known technical solutions. Based on this, it is concluded that the claimed technical solution meets the criterion of "inventive step".

Below is an option for the production of cold rolled steel using the proposed method of heat treatment.

Example. 08ps steel is smelted in a converter with a capacity of 160 tons, poured into continuously cast ingots 200-250 mm thick, hot rolled slabs into strips 2.5-4.2 mm thick, cold rolled to 0.7-1.5 mm thick, thermal processing in bell-shaped gas single-stage furnaces and training with compression 1.1-1.4%.

The heat treatment mode and the results of mechanical tests of experimental steel melts are given in the table.

The application of the invention improves the performance of bell-type furnaces by reducing the annealing time and reducing energy consumption while ensuring the required level of mechanical properties, as well as improving the quality of rolled products from carbon sheet steel for general purposes by reducing the number of surface defects "kink" and "welding sticking spots".

Table
Heat treatment mode and mechanical properties of experimental swimming trunks. Melting one 2 3 four 5 6 Mode new prototype new prototype new prototype Heating temperature, ° C 660 680 620 680 640 690 Mass of cages, t 60 60 90 90 120 120 The mass of the lower roll, t twenty twenty thirty thirty 40 40 The exposure time, h eighteen 28 22 24 26 thirty The total duration of heating and exposure, h 33 40 40 42 45 fifty Duration of cooling under a heating cap, h 5 - 3 - 5 - Productivity of heating furnaces, t / h 1.81 1,5 2.25 2.14 2.66 2,4 Natural blast furnace gas consumption, m ³ / t 161 196 117 137 105 125 Yield strength, σ _0.2, N / mm ² 215 230 225 200 225 220 Temporary resistance δ _in , N / mm ² 350 345 350 330 350 370 Elongation, δ ₈₀ ,% 38 38 34 35 36 37 Surface defects no no no kink no spots of adhesion, welding

Claims (1)

The method of heat treatment of coils of cold rolled sheet steel in bell-type furnaces, including heating in a protective atmosphere of a cage consisting of one or more rolls in a single-stage mode, holding, removing the cap and cooling under the muffle, unpacking and subsequent cooling in air, characterized in that the heating lead to a temperature of 620-660 ° C, the minimum exposure time of the rolls at a heating temperature is determined based on the mass of the lower roll in the cage according to the mathematical dependence τ _vyd = 14.65 + exp (-3.64 + 0.27 M _{lower roll} -0.00309 M ² _{lower roll} ), where τ _vyd - holding time, h; M _{lower steering wheel.} - mass of the lower roll in the charge, t, after the exposure time, turn off the heating, produce cooling under a heating hood to a temperature of 550-610 ° C, and cooling under a muffle is carried out to a temperature not exceeding 140 ° C.