RU2615667C1 - Method of producing hot-rolled sheets of low-alloyed steel of k65 strength grade for longitudinal electric-welded pipes - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: incessantly-cast subproduct of steel containing, wt %: carbon 0.04-0.07, silicon 0.15-0.35, manganese 1.45-1.60, chromium 0.22-0.30, nickel 0.24-0.30, copper not more than 0.15, titanium 0.015-0.030, vanadium 0.050-0.065, niobium 0.035-0.050, molybdenum 0.23-0.30, nitrogen not more than 0.007, aluminium 0.020-0.060, sulfur not more than 0.002, phosphorus not more than 0.012, iron and unavoidable impurities are the rest, is heated to a temperature of 1190±10 °C, subjected to rough rolling with specified reduction into an intermediate thickness, multiple of 4- thickness of the finished sheet, roll is cooled down, finish rolling is carried out at the start temperature of 720-745°C and the finish temperature of 755-770°C, rapidly cooled down into the controlled cooling set to temperature of 510-550°C, and then in the air with sheet structure mainly consisting of ferrite and bainite.

EFFECT: provision of satisfactory fracture strength of steel at a temperature of minus 5 degrees.

4 cl, 3 tbl

Description

The invention relates to the field of metallurgy, in particular to the production on a reversible mill of thick sheets with a thickness of more than 34 mm for the manufacture of pipes for high pressure gas pipelines.

A known method of producing sheet metal from low alloy pipe steel of strength class K65 with a thickness of 23-28 mm, including austenization at a temperature of 1180-1210 ° C of a slab billet of steel with the following ratio of elements, wt. %:

carbon 0.05-0.07 manganese 1,60-1,70 silicon 0.20-, 032 sulfur 0.001-0.002 phosphorus 0.003-0.012 nickel 0.15-0.25 chromium 0.15-0.25 copper 0.10-0.20 aluminum 0.025-0.045 niobium 0.075-0.095 titanium 0.01-0.02 vanadium 0.01-0.03 molybdenum 0.15-0.25 nitrogen 0.001-0.006 iron rest,

with carbon equivalent С _equiv <0.45% and with the parameter of resistance to cracking during welding Р _st ≤ 0.23%, preliminary deformation with regulated compressions of at least 12% at a temperature of 1000-1050 ° С, cooling of the obtained workpiece to the final start temperature deformation, its final deformation in a given temperature range, accelerated cooling of sheet metal to a regulated temperature and then slow cooling in a stack to ambient temperature, while the final deformation begins for the final thickness of sheet metal from 23 to 25 mm inclusive at a temperature of 810 ± 15 ° C, and for the final thickness of sheet metal from 25.1 to 28 mm inclusive at a temperature of 800 ± 20 ° C and complete for the final thickness of sheet metal from 23 up to 25 mm inclusive at a temperature of rolling end of 810 ± 10 ° С, and for a final thickness of sheet metal from 25.1 to 28 mm inclusive - at a temperature of end of rolling 790 ± 10 ° С, and accelerated cooling of sheet metal is carried out sequentially in two stages - turbulent water jets with surface cooling sheet metal to a temperature of 675 ± 10 ° C at a speed of 20-30 ° C / s, and then by laminar jets of water for sheet metal with a thickness of 23 to 25 mm inclusive to a temperature of the end of cooling of 505 ± 15 ° C at a speed of 17-23 ° C / s, and for sheet products with a thickness of 25.1 to 28 mm inclusive - to a temperature of the end of cooling of 495 ± 15 ° C at a rate of 18-24 ° C / s (RF patent No. 2492250, IPC C21D 8/02, C22C 38 / 38, C22C 38/42, publ. September 10, 2013).

The disadvantage of this method is the difficulty of obtaining sheets of strength class K65 with a thickness of more than 28 mm, which limits the possibility of their use for the production of large diameter pipes for main pipelines with a working pressure of up to 11.8 MPa.

Closest to the proposed method is the production of plate products of strength classes K65, X80 and L555 for the manufacture of electric-welded pipes of main pipelines, which includes the production of continuously cast billets from steel, austenitization of continuously cast billets by heating in a furnace, preliminary deformation, bending rolled to the temperature of the start of finish rolling, finishing rolling and subsequent regulated accelerated cooling of finished products with final delayed cooling and / or eye by cooling by air to ambient temperature, characterized in that the preform is obtained from steel with the following ratio of elements, wt. %:

carbon 0.03-0.08 silicon 0.12-0.35 manganese 1.65-2.10 chromium 0.01-0.30 nickel 0.01-0.40 copper 0.01-0.30 molybdenum 0.01-0.30 aluminum 0.02-0.05 niobium 0.03-0.09 vanadium 0.001-0.10 titanium 0.010-0.035 sulfur 0.0005-0.003 phosphorus 0.002-0.015 nitrogen 0.001-0.008 iron and inevitable impurities rest,

moreover, the ratio between the content of manganese, chromium, copper, silicon, nickel, molybdenum, vanadium, niobium, carbon and nitrogen in accordance with the ratios:

0.08 <(Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 <0.16,

-2.7 <log [Nb] [C + 8N] <- 2.0,

Cr + Ni + Cu + Mo <0.8,

moreover, austenitization is carried out at a temperature of not less than 100 ° C below the temperature Ts (TiN) of the dissolution of titanium nitrides, in accordance with the ratio:

Ts (TiN) = 14400 / (5.0-log [Ti] [N]),

where Ti and N are the content of titanium and nitrogen in steel, wt. %

and not lower than the temperature Ts (Nb (C, N)) of dissolution of niobium carbonitrides in accordance with the ratio:

Ts (Nb (C, N)) = (10100-27 [Mn] +200 [Si]) / (4.85-log [Nb] [C + 8N]),

where Nb and C are the content of niobium and carbon in steel, wt. %

and the time t holding in the languid zone is carried out in accordance with a given equation, while the preliminary stage of deformation is carried out so that in its last four passes, the relative compression increases in accordance with the ratio:

ε _i = (1.05 ... 1.35) × _εi-1 ± 2,%,

where ε _i and ε _i-1 - compression in the next and previous pass, respectively,

and accelerated cooling of the finished steel is carried out after preliminary editing of the rental, and the temperature of the beginning of cooling T _but is determined from the ratio:

T _but = 977-54Mn-102Ni-20Mo-866C-2.2Vcool ± 30, ° С,

where Vokhl is the cooling rate of rolled products from the end of rolling to the beginning of accelerated cooling, ° C / s,

and the temperature interval between the temperature of completion of rolling T _KP and the temperature of the beginning of accelerated cooling T _but is determined from the ratio:

Δ = -2.5Н + 92 ± 20, ° С,

where N is the thickness of the sheet, mm

(RF patent No. 2549023, IPC C21D 8/02, B21B 1/24, C22C 38/50, published on 04/20/2015).

The disadvantage of this method is the difficulty of obtaining sheets of steel of strength class K65 with a thickness of over 34 mm. Also, the crack resistance of the rolling stock is not guaranteed when testing crack opening at the crack tip (CTOD).

The technical result of the invention consists in producing sheets with a thickness of more than 34 mm from low-alloy steel of strength class K65 with satisfactory crack resistance at minus 5 ° C for electric-welded longitudinal joints for high-pressure gas pipelines.

The technical result is achieved by the fact that in the method for the production of hot rolled sheets of low alloy steel, including heating continuously cast billets above Ac ₃ , rough rolling with regulated compression into a roll of intermediate thickness, curing, finishing rolling, accelerated cooling in a controlled cooling installation to a temperature of 510-550 ° C and further cooling in air, continuously cast billets are obtained from steel with the following content of elements, wt. %:

carbon 0.04-0.07 silicon 0.15-0.35 manganese 1.45-1.60 chromium 0.22-0.30 nickel 0.24-0.30 copper no more than 0,15 titanium 0.015-0.030 vanadium 0,050-0,065 niobium 0.035-0.050 molybdenum 0.23-0.30 nitrogen no more than 0,007 aluminum 0,020-0,060 sulfur no more than 0,002 phosphorus no more than 0,012

iron and inevitable impurities rest,

before rolling, continuously cast billets are heated to a temperature of 1190 ± 10 ° С, then they are rough-rolled to a thickness of a roll that is a multiple of 4 thicknesses of the finished sheet, finishing rolling is started at a temperature of 720-745 ° С and completed at a temperature of 755-770 ° С, the structure of the finished sheet mainly consists of bainite and ferrite. The proportion of ferrite in the structure of the finished sheet is from 10 to 20% in the field of view with a diameter of 0.8 mm with an increase of 500 times, while the grain size of the ferrite is not more than 8 microns. The structure of the finished sheet may contain perlite in an amount of up to 5%.

The invention consists in the following.

According to the proposed method, a continuously cast billet is made of steel with a given chemical composition. The content of chemical elements in these ratios provides the necessary mechanical properties of the sheets when implementing the proposed technological regimes.

To obtain the required strength, the carbon content should be at least 0.04%, while its addition in an amount of more than 0.07% leads to a deterioration in the plastic properties of steel.

The addition of silicon is necessary for the deoxidation of steel during smelting. To ensure the necessary level of deoxidation, its content should be at least 0.15%, but not more than 0.35% to limit the amount of silicate inclusions that impair toughness and crack resistance.

Manganese increases the degree of saturation of ferrite with the dissolved elements involved in the dispersion hardening mechanism. For his. rational use, the manganese content should be at least 1.45%. When the manganese content is more than 2.0%, the toughness of steel is reduced.

In the proposed method, the effect of solid solution hardening of chromium is used. With an increase in the concentration of chromium, the hardenability of steel increases, and it becomes possible to form martensitic structures, which lead to a decrease in crack resistance, therefore, the upper limit of the chromium content is limited to 0.3%.

To increase the stability of austenite, nickel and copper are added to the steel. To obtain the desired effect, the nickel content should not be less than 0.24%. It is not economically feasible to add more than 0.30% nickel. To save nickel, steel is alloyed with copper. When the copper content is more than 0.15%, hot cracks may occur during rolling.

Titanium is a nitride-forming element that contributes to the grinding of grains with a content of more than 0.015%, but its addition in large quantities leads to a significant deterioration in crack resistance due to the formation of carbides, therefore, the upper limit of its content is limited to 0.030%.

Vanadium is a carbonitride-forming element that increases strength. Its addition in an amount of less than 0.05% is not effective, however, the addition of vanadium more than 0.065% leads to a deterioration in crack resistance when testing the opening at the crack tip.

Niobium is also a carbonitride-forming element that increases strength. Niobium carbides inhibit grain enlargement, contribute to the formation of a finely dispersed structure due to inhibition of recrystallization during finish rolling. A niobium content of less than 0.035% does not provide sufficient dispersion and grain-boundary hardening, an increase in the niobium content of more than 0.050% is not economically feasible.

Molybdenum increases the hardenability of steel. When adding molybdenum of less than 0.023%, the required hardenability index for steel is not achieved, and adding it in an amount of more than 0.30% leads to a significant increase in the cost of steel, which is not economically feasible.

Nitrogen is needed to isolate finely dispersed nitrides in order to reduce the diameter of austenitic grains. When the nitrogen content exceeds 0.007%, its concentration in the solid solution increases, which affects the toughness and crack resistance of steel at low temperatures.

Aluminum deoxidizes and modifies steel, binds nitrogen to nitrides. To reduce the oxygen content in the molten steel, at least 0.02% of aluminum must be added. When its content is more than 0.06%, the properties of steel in terms of viscosity and crack resistance are reduced.

With a sulfur content of more than 0.002%, sulfide inclusions are formed in the steel, which significantly reduce the toughness and crack resistance.

Phosphorus is one of the elements that are most prone to segregation and the formation of segregation along grain boundaries, and, as a result, adversely affect the toughness of steel and crack resistance, therefore, the upper limit of the phosphorus content is set no more than 0.012%.

The main feature of the proposed technology is to obtain a homogeneous structure at each stage of the process.

Before rolling, the continuously cast billet is heated to a temperature of 1180-1200 ° C. Exceeding the upper boundary of the interval stimulates an abnormal growth of austenite grains, leading to a decrease in the strength and viscosity properties of rolled products. If the lower limit of the heating temperature range is not reached, carbonitrides do not dissolve well in austenite, this has a negative effect on the course of recrystallization processes, and also reduces the strength and viscous properties of rolled products.

To ensure satisfactory test results with a falling load, taking into account the increased thickness of the rolled products, it is necessary to ensure the thickness of the intermediate roll at least 4 thicknesses of the finished sheet. Obtaining a fivefold intermediate roll in thickness reduces the total degree of deformation at the rough rolling stage, hindering the required grinding of austenite grain.

The temperature range of the beginning and end of deformation at the finishing stage of rolling is chosen based on the need to prepare austenite for subsequent transformation by creating deformed austenite grains containing deformation bands and having a high dislocation density. The rational temperature range of the beginning of the finish rolling is defined as the interval 720-745 ° C, the end - 755-770 ° C.

Accelerated cooling has a positive effect on the strength and viscoplastic properties of finished steel. The temperature interval for the end of accelerated cooling of 510-550 ° C provides the desired ferrite-bainitic structure. A higher temperature at the end of accelerated cooling leads to an increase in the perlite fraction in the structure, and a lower one - bainite.

The proposed method for the production of hot-rolled sheets leads to the formation of 10 to 20% ferrite in the structure of finished products in the field of view with a diameter of 0.8 mm with an increase of 500 times and up to 5% perlite, while the average grain size of the ferrite does not exceed 8 microns.

Implementation examples.

For the experiment, five heats of various chemical composition were smelted (Table 1). Swimming trunks 1, 2 and 3 are made in accordance with the claims.

After heating the slabs with a thickness of 313 mm to a temperature of 1180-1200 ° C, a rough rolling stage was carried out to a roll thickness of 175-177 mm, after which the finishing stage of rolling was carried out at temperatures of 720-770 ° C to a thickness of 36.5 mm, then accelerated cooling to temperatures of 510-550 ° C with final cooling in air to ambient temperature.

Static tensile tests were carried out on flat full-size samples according to GOST 1497, made from samples taken in the transverse direction relative to the rolling direction. Dynamic tests with a vertically falling load were carried out on samples with a V-shaped notch at a temperature of minus 5 ° C according to GOST 30456; on a pendulum headstock - on samples with a V-shaped notch at a temperature of minus 10 ° С according to GOST9454; crack resistance tests - at a temperature of minus 5 ° С according to BS7448.

Implementation options of the proposed method and test results are shown in tables 2 and 3, respectively.

The test results showed that, according to the invention, the method of production of steel of the proposed chemical composition (options No. 1-3) provides a satisfactory level of mechanical properties. With the prohibitive values of the proposed modes (options No. 4-10), it is not possible to achieve the required level of strength, ductility, steel toughness and crack resistance.

Thus, the application of the described rolling method ensures the achievement of the required results, namely, the production of rolled products with a thickness of more than 34.0 mm from low-alloy steel of strength class K65 with satisfactory crack resistance at minus 5 ° C for electric-welded straight-seam pipes for high-pressure gas pipelines.

Claims (6)

1. A method for the production of hot-rolled thick sheets of low alloy steel, including heating a continuously cast billet above Ac ₃ , rough rolling with regulated compression into a roll of intermediate thickness, reinforcing it, finishing rolling, accelerated cooling in a controlled cooling installation to a temperature of 510-550 ° C and further air cooling, characterized in that continuously cast billets are obtained from steel with the following content of elements, wt. %: carbon 0.04-0.07 silicon 0.15-0.35 manganese 1.45-2.00 chromium no more than 0.30 nickel 0.24-0.30 copper no more than 0,15 titanium 0.015-0.030 vanadium 0,050-0,065 niobium 0.035-0.050 molybdenum 0.23-0.30 nitrogen no more than 0,007 aluminum 0,020-0,060 sulfur no more than 0,002 phosphorus no more than 0,012 iron and inevitable impurities rest, heating of the continuously cast billet is carried out to a temperature of 1190 ± 10 ° C, rough rolling of the billet is carried out with an intermediate thickness of the roll, a multiple of 4 thicknesses of the finished sheet, while finish rolling is started at a temperature of 720-745 ° C and completed at a temperature of 755-770 ° C obtaining a structure consisting mainly of bainite and ferrite. 2. The method according to p. 1, characterized in that the proportion of ferrite in the structure of the finished sheet is from 10 to 20% in the field of view with a diameter of 0.8 mm with an increase of 500 times. 3. The method according to p. 1, characterized in that in the structure of the finished sheet may be perlite in an amount of up to 5%. 4. The method according to p. 1, characterized in that in the sheet structure the average grain size of the ferrite is not more than 8 microns.