RU2353671C2 - Method for production of thermomechanically treated hot rolled pipes - Google Patents

Abstract

FIELD: technological processes, metal working.

SUBSTANCE: invention is related to pipe rolling. In order to increase impact elasticity, on completion of hot plastic deformation pipe is cooled with the rate that prevents decay of austenite down to temperature that lies in the range of martensite transformation, and then cooled on air and heated in inductor supplied with AC of 50…60 Hz frequency, down to temperature of 740-760°C for 1-1.5 min, maintained for 2-4 minutes and finally cooled on air.

EFFECT: increased impact elasticity of products.

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Description

The invention relates to heat treatment in pipe production.

A known method of thermomechanical processing of seamless pipes [USSR Author's Certificate No. 347355, Cl. C21D 9/08, 1972], which is essentially one of the varieties of high-temperature thermomechanical processing (HTMO), since during its implementation the pipes, the material of which is in the state of austenite, are subjected to controlled cooling after hot plastic deformation. In fact, according to the invention, this method includes hot plastic deformation of pipes and cooling, consisting of initial, performed at a speed that prevents the decomposition of austenite, and then final in air. According to the inventors, such continuous two-stage cooling allows to obtain a bainitic structure that provides high strength and satisfactory ductility.

However, not all steels under continuous accelerated, albeit two-stage, cooling give a stable bainitic transformation, which can be guaranteed only by isothermal quenching. So, with continuous accelerated cooling of steels (for example, high carbon and some alloyed), for which the points of the onset of martensitic transformation and the onset of bainitic transformation are close at 450 ... 500 ° C, the bulk of the initial austenite is converted to martensite, and only a small part of the residual austenite is transformed in bainite, i.e. a mixed martensitic-bainitic structure is formed, which has high ductility and low ductility and toughness [Metallurgy and heat treatment of steel: Handbook in 3 vols. Ed. Bernstein M.L. and Rakhstadt A.G. T.2. - Basics of heat treatment. M .: Metallurgy, 1983, p.150-153].

The technical task is set: to increase the toughness of a hot-rolled pipe.

The problem is solved by creating a method for the production of thermomechanically processed hot rolled pipes, including hot plastic deformation of the pipe, its cooling at a rate that prevents the decomposition of austenite, and subsequent cooling in air, in which according to the invention, cooling at a speed that prevents the decomposition of austenite is carried out to a temperature lying in the range of martensitic transformation, and after cooling in air, the pipe is heated in an inductor fed by alternating current with a frequency of 50 ... 60 Hz, to a pace Aturi 740 ... 760 ° C for 1.5 ... 1 min, incubated for 2 ... 4 min and finally cooled in air.

The use of the induction heating method provides high-speed and uniform heating of the pipes in volume and thereby high productivity, and the simplicity of the design of induction plants using a mains current of 50 ... 60 Hz for power supply guarantees a minimum of capital costs. In addition, the use of a current frequency of 50 ... 60 Hz allows you to limit the heating temperature to 740 ... 760 ° C. This temperature is slightly higher than the permissible temperatures for ordinary furnace tempering, since it exceeds critical values, however, due to the high heating rate and subsequent short exposure, the transformations in the pipe material are delayed and do not go to a critical stage, which results in an effect similar to furnace tempering. As a result, the structure of tempering sorbitol is formed, which ultimately guarantees sufficient strength of the processed material with its high ductility and viscosity.

By changing the exposure time at a given temperature, the required values of hardness and strength are obtained. Thus, reducing exposure time, provide increased hardness and strength. Increasing the exposure time reduces the hardness and strength.

It is known that the cooling rate that prevents the decomposition of austenite, also called supercritical speed, is different for different steels and is determined by their chemical composition. In particular, for steels of the martensitic class (for example, 40X13) this speed is ensured by conventional cooling in air, i.e. continuous cooling in air also turns out to be the two-stage that is used in the proposed method, since first, pearlite decomposition of austenite is blocked during cooling from the temperature of the end of hot plastic deformation, and then, after the temperature drops below the start point of the martensitic transformation, austenite is transformed into martensite. For other steels, the supercritical cooling rate is achieved by choosing a cooling medium, which can be used as a moist (water-air), water, oil, emulsion, as well as a powerful stream of directed air.

Example 1. A hot-rolled pipe made of steel 45X, having an outer diameter of 92 mm, a length of 1100 mm and a wall thickness of 13 mm, after it exited the calibration mill, was cooled from a temperature of 880 ° C, at which steel 45X has an austenitic state, to a temperature of 260 ° C at a rate of 31 ° C / s, which for this steel is guaranteed to exclude the decomposition of austenite. This cooling rate was provided by holding the pipe in water having a temperature of 35 ° C for 20 s, after which the pipe was cooled in air to a temperature of not more than 60 ° C. Then the pipe was heated to a temperature of 740 ° C for 1 min and kept at this temperature for 3 min, heating and holding by translational movement of the pipe at a speed of 0.017 m / s through an inductor with a diameter of 180 mm and a length of 4 m, fed by alternating current frequency 50 Hz at a voltage of 380 V. After exiting the inductor, the pipe was cooled in air to ambient temperature.

The mechanical properties of the finished pipes are as follows: yield strength - 510 ... 540 MPa, tensile strength - 720 ... 760 MPa, elongation - 22 ... 25%, impact strength at a temperature of 20 ° C - 1.4 ... 1.6 MJ / m ² and at -40 ° C - 0.7 ... 0.9 MJ / m ² . The result shows that the pipes can be successfully operated in conditions of freezing temperatures.

Example 2. A pipe made of steel 30XMA with a diameter of 102 mm with a wall thickness of 7 mm and a length of 1500 mm after it exited the calibration mill was cooled by the air flow created by a 25 kW fan from a temperature of 880 ° C to a temperature of 260 ° C for 50 s, which ensured a cooling rate of 12.5 ° C / s, then it was cooled in calm air to a temperature not exceeding 60 ° C. Next, the pipe was heated to a temperature of 760 ° C in 1 min and kept at this temperature for 4 min by translational movement at a speed of 0.013 m / s through an inductor with a diameter of 180 mm and a length of 4 m, powered by an alternating current of 50 Hz at a voltage of 380 V, after which it was finally cooled in air.

Pipe mechanical properties: yield strength - 630 ... 650 MPa, tensile strength - 920 ... 950 MPa, elongation - 20 ... 22%, impact strength - 0.9 ... 1.2 MJ / m ² . This means that the effect of reversible temper embrittlement has been suppressed.

Claims (1)

A method for the production of thermomechanically processed hot-rolled pipes, including hot plastic deformation of the pipe, cooling at a rate that prevents the decomposition of austenite, and subsequent cooling in air, characterized in that the cooling at a rate that prevents decomposition of austenite is carried out to a temperature lying in the range of martensitic transformation, and after cooling in air, the pipe is heated in an inductor fed by alternating current with a frequency of 50-60 Hz to a temperature of 740-760 ° C for 1-1.5 minutes, kept for 2-4 min and finally cooled in air.