RU2266341C1 - Method for thermal processing (improvement) of composite roller band - Google Patents

Abstract

FIELD: manufacture of composite roller bands for continuous casting machine.

SUBSTANCE: method involves first step including heating band to temperature of 580-620 C and holding at such temperature for 0.8-1.2 hours, and second step including heating band to hardening temperature of 930-980 C and holding at such temperature for 2.4-2.6 hours; providing tempering by heating to temperature of 660-720 C and holding at such temperature for 4-6 hours.

EFFECT: increased resistance of band to thermal abrasion and improved quality of cast blank surfaces.

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Description

The invention relates to the field of heat treatment and can be used in the manufacture of bandages of composite rollers of continuous casting machines.

The bandaged rollers of continuous casting machines (CCM) work in constant contact with the cast metal, as a result of which they are exposed to thermocyclic loads, causing intense thermal wear of the bandages. To increase the strength and wear resistance of the bandage rollers CCM blanks are made of chromium-molybdenum-vanadium steel.

A known method of manufacturing a roll of chrome-molybdenum-vanadium steel, according to which, after machining, the product is subjected to heat treatment, including heating to austenitization temperature, water quenching and low-temperature tempering [Application No. 2205656 (Japan), IPC C 22 C 38/00, 21 V 27 / 00 1990].

The disadvantage of this method is that it is not suitable for heat treatment of the bandage of the roller caster, because After quenching and low-temperature tempering, the product retains high residual stresses, which provoke its thermal wear.

The closest in its technical essence and the achieved results to the proposed invention is a method of heat treatment of a roll, comprising heating it to a hardening temperature in two stages: first to a temperature of 600-650 ° C, and then to a hardening temperature of 840-850 ° C. After hardening, the product is subjected to tempering at a temperature of 170-190 ° C, and the exposure time at the tempering temperature is assigned based on: 2 min per 1 mm of barrel diameter [V.P. Polukhin et al. Reliability and durability of cold rolling rolls. M .: Metallurgy, 1976, p.184-186, 196-197] - prototype.

The disadvantages of this method are that after such heat treatment, products made of chromium-molybdenum-vanadium steel have low resistance to thermal wear, which negatively affects the durability of the shrouded casters of the caster and the surface quality of cast billets.

To solve the technical problem in the known method of heat treatment of the composite roller bandage, including heating it to a quenching temperature in two stages, quenching and subsequent tempering, at the first stage, the bandage is heated to a temperature of 580-620 ° C, at which it is held for 0.8 -1.2 hours, at the second stage - up to a quenching temperature of 930-980 ° С, at which they are kept for 2.4-2.6 hours, and tempering is carried out by heating to a temperature of 660-720 ° С and holding at this temperature within 4-6 hours

The invention consists in the following. The heat treatment (improvement - hardening + tempering) of the bandage should provide both its high strength, resistance to thermal wear, and also eliminate the formation of cracks in the heating processes for hardening and hardening.

Heating at the first stage to a temperature of 580-620 ° C with a shutter speed of 0.8-1.2 h provides phase stress relief in chrome-molybdenum-vanadium steel and reduces the required holding time at higher temperatures at the second stage to equalize the temperature field of the bandage. Further heating to a quenching temperature of 930–980 ° C with a holding time of 2.4–2.6 h ensures complete austenitization of the steel and the formation of a fine-grained microstructure necessary for subsequent quenching. In addition, during the exposure time of 2.4-2.6 hours at a reheating temperature, oxidation and weakening of grain boundaries do not have time to develop in the metal matrix of the steel.

In the hardened state, chrome-molybdenum-vanadium steel is characterized by high thermal and phase stresses and an unstable phase composition. High temperature tempering at 660–720 ° С with holding for 4–6 h leads to the decomposition of martensite formed upon quenching from a temperature of 930–980 ° С, and the formation of a cellular microstructure of the matrix. In the tempered condition, the steel has a stable fine-grained microstructure, which is characterized by high thermal stability and does not undergo changes during thermal cycles during operation of the bandaged roller. Due to this, an increase in the resistance of the bandage against thermal wear, as well as the surface quality of cast billets, is achieved.

It was experimentally established that with a decrease in the heating temperature in the first stage below 580 ° C or a decrease in the exposure time of less than 0.8 hours, a decrease in the phase stresses of the steel is not achieved, and to equalize the temperature field of the bandage, its longer exposure at elevated temperatures in the second stage of heating is required. This adversely affects the resistance of the bandage. An increase in the heating temperature above 620 ° C or a holding time of more than 1.2 h increases the heterogeneity of the microstructure and reduces the resistance of the bandage.

At a heating temperature for hardening below 930 ° C and a holding time of less than 2.4 hours, chromium-molybdenum-vanadium steel retains a two-phase structure, therefore, after hardening it has low strength and wear resistance. An increase in the reheating temperature above 980 ° C and the exposure time at this temperature for more than 2.6 hours contributes to the excessive growth of austenitic grains, the weakening and oxidation of their boundaries. As a result, the operational stability of the bandage and the surface quality of cast billets are reduced.

It was also experimentally established that with a decrease in tempering temperature below 660 ° C or a decrease in tempering time of less than 4 hours, complete removal of the structural and phase stresses of quenching is not achieved. The bandage acquires uneven mechanical and tribological properties, which leads to its increased thermal wear. An increase in tempering temperature of more than 720 ° C and a holding time of more than 6 hours leads to a drop in the hardness, strength and wear resistance of the bandage, deterioration of the surface quality of cast billets, which is unacceptable.

Method implementation examples

The bandage of a composite roller made of chromium-molybdenum-vanadium steel grade 25X1M1F in the form of a pipe segment with an external diameter of 600 mm and a wall thickness of 30 mm is heated in the first stage to an intermediate temperature T _p = 600 ° C, at which it is kept for a time τ _p = 1.0 h. Then the bandage is heated to a quenching temperature T _s = 950 ° C and, after holding for a time τ _s = 2.5 hours, is quenched in an oil bath.

The hardened bandage is released by holding it at a temperature of T _o = 690 ° C for τ _o = 5 hours, after which it is cooled in air.

Heat-treated (improved) bandages are pressed onto the axis and installed on a horizontal section of the radial slab caster. During operation of the continuous casting machine, the composite roller bandage along which the cast slab moves is subject to cyclic thermal and mechanical stresses. Due to the fact that in the process of thermal improvement, a fine-grained cellular microstructure of the bandage was formed and phase, structural and thermal stresses were removed, the bandage has high strength and resistance to thermal wear. Due to this, the operating time of the bandaged roller on the slab continuous casting machine was: Q = 2.5 thousand pieces of spilled melt with a high surface quality of the slabs.

The options for implementing the method of heat treatment of the composite roller brace are presented in the table.

The table shows that when implementing the proposed method (options 2-4), an increase in the resistance of the bandage against thermal wear is achieved. In the case of transcendental values of the declared parameters (options 1 and 5) and the implementation of the prototype method (option 6), the resistance of the bandage against thermal wear is reduced, as a result of which the surface quality of the cast slabs is deteriorated.

The technical and economic advantages of the proposed method consist in the fact that the proposed heat treatment (improvement) regimes provide the formation of the microstructure of the composite roller band from chromium-molybdenum-vanadium steel, which has high strength and is most resistant to thermal wear. This allows you to increase the number of melts of the metal cast on the continuous casting machine without replacing the bandaged rollers with a high surface quality of the slabs.

As a basic object in determining the effectiveness of the proposed method adopted the prototype method. Using the proposed method will increase the profitability of the production of continuously cast slabs in continuous casting machines by 2.5-4.4%.

Table
Heat treatment modes and resistance of the bandage No. p / p T _p , ° C τ _p , h T _s , ° C τ _s , h T _o , ° C τ _about , h Q, thousand swimming trunks 1. 570 0.7 925 2,3 655 3.9 0.97 2. 580 0.8 930 2,4 660 4.0 2.20 3. 600 1,0 950 2,5 690 5,0 2,50 4. 620 1,2 980 2.6 720 6.0 2.27 5. 625 1.3 990 2.7 730 6.2 1.32 6. 630 not regl. 850 not regl. 190 30,0 0.99

Claims (1)

A method of heat treating a composite roller bandage, including heating it to a hardening temperature in two stages, hardening and subsequent tempering, characterized in that in the first stage the bandage is heated to a temperature of 580-620 ° C, at which it is held for 0.8-1, 2 hours, at the second stage - up to a quenching temperature of 930-980 ° С, at which they are kept for 2.4-2.6 hours, and tempering is carried out by heating to a temperature of 660-720 ° С and holding at this temperature for 4 -6 hours