RU1782241C - Method of heat treatment using the heat of hot rolling process - Google Patents

Abstract

The inventive surface of the hot rolling surface of the reinforcing bar is cooled below the point Mn to a depth of 0.3-0.5 mm at a speed of V (2.4 / 0: 104 ± 15) ° C / s, where D is the diameter reinforcement, mm, and heated to MH + (200-300) ° C over a period of so-called determined from the ratio of (1.3-0.0583D) with g 0.9 s. After which the hire is finally cooled. Using this method can significantly increase the resistance of the reinforcement to stress corrosion cracking ..1 table. „

Description

The invention relates to ferrous metallurgy, in particular to heat treatment, reinforcing steel using the heat of rolling heat and can be used in the manufacture of high strength bar reinforcement.

Known methods of heat treatment of rolled products. For example, there is a known method of heat treatment of rolled products, mainly wire rod, using rolling heat, comprising cyclic cooling to 650-500 ° C with surface supercooling at a depth of 0.05-0.3 mm below the point Mn during each cycle, moreover, cooling the second and subsequent cycles are performed when the surface of the rolled product reaches 650-500 ° C.

The closest to the claimed method in technical essence and the achieved positive result is a method of heat treatment of rolled products, mainly rod reinforcement, using the heat of rolling heating.

including cyclic cooling for 1-2 s with the number of cycles equal to two and surface supercooling at a depth of 0.15-0.2 R below the point Mn during each cycle with intermediate heating to Mn + (5-20) ° C and final by heating the surface to Mn + (YuO-250) ° C and final cooling, where R is the radius of the bar reinforcement.

A disadvantage of the known methods is that they do not provide high tensile reinforcement having high resistance to stress corrosion cracking.

An object of the present invention is to increase the resistance to stress corrosion cracking under high tensile strength reinforcing bars.

This goal is achieved by the fact that in the known method of heat treatment of bar reinforcement, including supercooling the surface below the point Mn with intermediate heating of the surface and

4 00 S

Yu

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final cooling, according to the invention, in the first cycle, subcooling below Mn is carried out to a depth of 0.3-0.5 mm with a cooling rate determined from the ratio

V- {2 4 / D-104 ± 150) ° С / s intermediate surface heating after the first cooling cycle is carried out for a period of time, the minimum duration of which is determined from the ratio

t (1.3-0.05830) s,

and the maximum does not exceed 0.9 s, while the temperature on top of bst is raised to

MH + (200-300) ° C, where D is the diameter of the rod;

V is the cooling rate, mm / s;

t-time, s;

Mn is the martensitic transformation temperature.

The limits of the technological parameters of the claimed method were chosen because, at a depth of the primary martensitic layer less than 0.3 mm, the influence of the secondary layer is so great that high corrosion resistance under stress is not provided at a depth of the martensitic layer above 0, 5 mm of the amount of heat remaining in the core of the hardenable steel is insufficient to heat the surface, which reduces the corrosion resistance of the steel To ensure uniform thickness of the martensitic layer over large diameters cooling should be not be more than

V (2.4 / D “10 +150) ° C / s

At a cooling rate lower than

At (2.4 / D 104-150) ° C / s, a significant amount of heat will be attributed to the core thickness required for the formation of the martensitic layer from the core of the rod so that the remaining heat is not enough to obtain the structure of highly tempered martensite in the surface layer, which reduces the corrosion resistance of the car. Intermediate heating of the surface to a temperature below MH + 200 ° C does not provide the necessary degree of tempering of the primary martensitic layer necessary for resistance to stress corrosion cracking. Intermediate heating of the surface to temperatures above MH + 300 ° C is impractical from the viewpoint of the manufacturability of the process for additional time and production high mechanical properties The duration of the intermediate heating of the surface should not exceed 0.9 s, since with a longer time in

the core of the core from supercooled austenite begins to precipitate crystals of excess ferrite, which adversely affects the mechanical properties. WITH

on the other hand, when the duration of the intermediate heating is less than (1.3-0.5830) s, the degree of martensite self-tempering that has begun does not ensure the corrosion resistance of the rolled product.

Example. In the small-grade workshop of the West Siberian Metallurgical Combine at the small-grade mill 250-1, experimental testing of the proposed method of heat treatment was carried out

rod reinforcement No. 14 from steel 28С of industrial melting (Мн 380 ° С). For this, billets with a section of 80x80 were heated to a temperature of 1200+ 20 ° C, rolled on a continuous small-grade mill 250-1 and

conducted cyclic cooling with supercooling of the surface below the point Mn during each cycle with intermediate heating of the surface. In this case, subcooling below the point Mn in the first cycle

led to a depth of 0.3-0.5 mm with a cooling rate of 1670-1850 ° C / s, and the duration of the intermediate heating of the surface was 0.5-0.9 s at a heating temperature of 590-670 ° C.

According to the proposed method, several modes were tested, involving changing the parameters of the thickness of the martensitic layer, cooling rates, pause durations in the declared range of their changes and beyond the boundary values. In addition, prototype tests were conducted. After the implementation of these modes according to the known method and prototype, the tensile strength, yield strength and time to failure of the sample were determined when testing the resistance to stress corrosion cracking.

Tests for resistance to stress corrosion cracking were carried out according to the method of accelerated stress tests in a boiling nitrate solution, which served as an aggressive corrosive medium and consisted of 600 parts by weight of

calcium nitrate (Ca (0z) 2). ammonium nitrate () and 350 parts by weight water. The temperature of the medium was provided in the range of 98-100 ° C using an electrocontact thermometer. The tests were carried out on lever plants, which made it possible to create a bend of samples 500 mm long and that provided a constant level of stresses in time within 0.3 0.2.

The corrosion cracking of steel was the time before failure of the specimen.

The obtained test results of the proposed method in comparison with the prototype are shown in the table.

From this table it can be seen that in the manufacture of heat-strengthened corrosion-resistant reinforcing steel by the proposed method, the resistance to stress corrosion cracking is 18 times higher. than steel obtained by a known method (prototype), which is taken as the base object.

The industrial test certificate of the claimed method is attached.

Using the proposed method of heat treatment of bar reinforcement will allow, in comparison with the prototype, to increase the operational properties of thermally hardened steel from low alloy steels, for example 28C. In addition, the proposed method allows to obtain high resistance to corrosion

Claims (1)

stress cracking on steels that do not contain scarce and expensive alloying elements. The claims A heat treatment method using rolling heat, comprising supercooling the surface below the point Mn, heating the surface to a predetermined temperature, and final cooling, characterized in that that, in order to increase the resistance to stress corrosion cracking, surface supercooling is carried out to a depth of 0.3-0.5 mm with a velocity V, determined from the ratio V (-104 ± 150) ° C / s surface heating is carried out to MN-K200-300) ° C for a time g determined from the mathematical expression (1.3-0.0583D) c, 9c, where D is the diameter of the rod, mm. Tests terminated after 220 hours