RU2583973C1 - Method of producing thick-wall pipe steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: to increase strength properties of sheets with thickness of 14-20 mm of pipe steel with strength class K60 at maintaining sufficient ductility and impact strength are produced continuously-cast slab thickness 300±20 from steel containing, wt%: carbon 0.06-0.08, silicon 0.25-0.40, manganese 1.60-1.70, sulphur not more than 0.003, phosphorus not more than 0.013, chromium not more than 0.08, nickel 0.20-0.30, copper 0.10-0.20, aluminium 0.025-0.045, nitrogen not more than 0.008, vanadium 0.020-0.035, titanium 0.015-0.030, niobium 0.040-0.055, iron and impurities is balance, wherein carbon equivalent is C_EQ ≤ 0.43, then slab is heated to temperature of 1,200-1,220 °C, rough rolling at 1,040±60 °C for 7-10 passes at reduction of not less than 12 % per pass by thickness of 95±15 mm, cooling of intermediate roll - 860±20 °C, finish rolling with total reduction of 75-85 %, rolling end temperature set 855±15 °C and accelerated cooling at a rate of 14-22 °C/s to temperature of 555±15 °C.

EFFECT: invention can be used for production of low alloyed pipe steels.

1 cl, 3 tbl, 1 ex

Description

The invention relates to the field of metallurgy, and more particularly to the production of sheet metal on a reversible plate mill, and can be used in the manufacture of thick sheets of low alloy steels using controlled rolling.

A known method for the production of cold-resistant sheet metal (see RF patent No. 2265067), including obtaining a billet of steel containing, wt.%: C = 0.04-0.1; Mn = 0.60-0.90; Si = 0.15-0.35; Ni = 0.10-0.40; Al = 0.02-0.06; Nb = 0.02-0.06; V = 0.03-0.05; the rest is iron and impurities. The method involves austenization of a workpiece at a temperature of 1100-1150 ° C, preliminary deformation (rough rolling) with a total compression of 35-60% at a temperature of 900-800 ° C, subsequent cooling of the intermediate workpiece (chilling) by 50-70 ° C, final deformation ( fine rolling) with a total degree of compression of 65-75% at a temperature of 830-750 ° C, accelerated cooling of sheet metal to a temperature of 500-260 ° C and slow cooling to a temperature of no higher than 150 ° C.

The disadvantages of this method include the fact that obtained when using it, a thick sheet of low alloy steel has insufficiently high strength properties. Values of yield strength and ultimate tensile strength claimed for this method comprise σ _m = 300-320 MPa, σ _in = 400-455 MPa. At the same time, the regulatory requirements for the steel pipe strength class K60 constitute σ _m = 510-610 MPa, σ _in = 590-700 MPa.

There is also a known method for the production of plate steel (see RF patent No. 2393236), including steelmaking, casting, heating and thermo-deformation rolling of the billet and accelerated cooling of the finished steel, while the steel has the following chemical composition, wt.%: Carbon 0.03-0 , 20, manganese 0.50-2.10, silicon 0.10-0.50, niobium 0.01-0.15, aluminum 0.01-0.10, titanium 0.005-0.05, nitrogen 0.002-0.012 , sulfur 0.0005-0.010, phosphorus 0.003-0.050, iron - the rest. Thermodeformation finish rolling in the temperature range of (Ar + _h 30 ° C) to (Ar _of -30 ° C), the subsequent rapid cooling is performed in two stages: the first stage at a rate of 10-30 ° / s to a temperature of 650-550 ° C, then after a pause of 3-10 s in the second stage at a speed of 5-20 deg / s to a temperature of 550-450 ° C. Subsequent cooling in air to 100 ° C is carried out slowly at a speed of 0.1-0.01 deg / s. The steel additionally contains one or more elements from the series, wt.%: 0.01-0.15 V; 0.05-0.50 Mo; 0.01-0.80 Ni; 0.01-0.80 Cr; 0.01-0.80 Cu, with carbon equivalent, C _equiv = 0.32-0.46.

The disadvantage of this method is the low proportion of the viscous component in the fracture of the steel sheet when tested with a falling load.

, охлаждают раскат до 760-800°С и подвергают чистовой прокатке с обжатиями не менее 12% за проход, за исключением трех последних проходов, затем готовый штрипс ускоренно охлаждают до температуры, определяемой в зависимости от его толщины из соотношения: $$T_{ко}=(422-\mathrm{0,1364}\cdot h_{ш}^2+\mathrm{3,6273}\cdot h_{ш})\pm 15°C$$

и замедленно охлаждают.The closest in technical essence is the method (see RF patent No. 2393239), according to which, to ensure high strength combined with high adaptability, ductility and cold resistance in strips with a thickness of 20-40 mm, the workpiece is obtained from steel with the following content of elements, wt.% : 0.03-0.06 C; 1.5-1.7 Mn, 0.15-0.35 Si; 0.15-0.3 Ni; 0.04-0.06 Nb; Cr≤0.2; 0.08-0.15 Mo; 0.15-0.3 Cu; 0.02-0.04 V; 0.005-0.02 Ti; 0.02-0.05 Al; iron and impurities, with the content of each element of the impurity less than 0.03% - the rest, while the carbon equivalent is C _equiv ≤0.4, then the workpiece is subjected to rough rolling at a temperature of 1000-920 ° C with a degree of reduction in the first two passes of not less than 9% per pass, and in subsequent no less than 12% per pass on the thickness of the roll, determined, depending on the thickness of the finished strip, from the ratio: $$H_{R\mathrm{but}\mathrm{from}\mathrm{to}}=(161.5+0.0955\cdot h_w^2-\mathrm{4,6191}\cdot h_w)\pm 5mm$$

, cool the roll to 760-800 ° C and finish rolling with a reduction of at least 12% per pass, with the exception of the last three passes, then the finished strip is rapidly cooled to a temperature determined depending on its thickness from the ratio: $$T_{\mathrm{to}\mathrm{about}}=(422-0.1364\cdot h_w^2+\mathrm{3,6273}\cdot h_w)\pm \mathrm{fifteen}°C$$

and slowly cool.

The disadvantage of this method is the poor performance of the toughness of the obtained hire.

The technical result of the invention is to increase the strength properties of economically alloyed tubular steel of strength class K60 while maintaining sufficient ductility and toughness of sheets with a thickness of 14-20 mm

The specified technical result is achieved by the fact that in the method for producing plate steel, including producing a continuously cast slab, heating it, rough rolling, cooling the intermediate roll before finishing rolling, finishing rolling, accelerated cooling of the finished roll to a predetermined temperature and subsequent slow cooling, difference from the closest analogue of a continuously cast slab with a thickness of 300 ± 20 mm is obtained from steel of the following chemical composition, wt.%:

carbon 0.06-0.08 silicon 0.25-0.40 manganese 1,60-1,70 sulfur no more than 0,003 phosphorus no more than 0,013 chromium no more than 0.08 nickel 0.20-0.30 copper 0.10-0.20 aluminum 0.025-0.045 nitrogen no more than 0,008 vanadium 0,020-0,035 titanium 0.015-0.030 niobium 0,040-0,055 iron and impurities rest

moreover, the carbon equivalent is C _equiv ≤0.43, while the slab is heated to a temperature of 1200-1220 ° C, rough rolling is carried out at a temperature of 1040 ± 60 ° C in 7-10 passes with a reduction ratio of at least 12% per pass per thickness 95 ± 15 mm, intermediate rolling is cooled to a temperature of 860 ± 20 ° C, finish rolling is carried out with a total degree of reduction of 75-85%, while the temperature of the end of rolling is set to 855 ± 15 ° C, accelerated cooling is carried out at a speed of 14-22 ° C / s to a temperature of 555 ± 15 ° C.

The invention consists in the following. First, a continuously cast slab (billet) is made of steel with a given chemical composition. The specified content of the elements provides the required value of the carbon equivalent, as well as the mechanical properties of the finished sheet when implementing the proposed technological processing modes.

The carbon content in the steel of the proposed composition determines its strength. A decrease in carbon content of less than 0.06% leads to a decrease in its strength below an acceptable level. An increase in carbon content of more than 0.08% worsens the plastic and viscous properties of plate products and leads to their unevenness due to segregation.

Additives of manganese and nickel within the claimed limits contribute to solid-solution hardening of the metal and, accordingly, increase the cold resistance and corrosion resistance of the finished product. A lower content of these elements does not allow to provide the required cold resistance, a larger one reduces weldability and is not economically feasible.

When the silicon content is less than 0.25%, the deoxidation of steel deteriorates, and the strength of sheet metal decreases. An increase in the silicon content of more than 0.40% leads to an increase in the number of silicate inclusions, and reduces the toughness of the metal.

The niobium additives within the specified limits serve the purpose of dispersion hardening, and also inhibit the growth of austenitic grain and contribute to the appearance of a subgrain structure during cooling, which is fixed and stabilized by dispersed carbide particles. When the niobium content is less than 0.040%, sufficient dispersion and grain-boundary hardening is not provided. An increase in the niobium content of more than 0.055% leads to a deterioration in the weldability of steel and is not economically feasible in view of the increase in alloying costs.

A vanadium content of more than 0.035% leads to a deterioration in the weldability of steel and is not economically feasible due to the increase in alloying costs. When the vanadium content is less than 0.020%, sufficient dispersion hardening is not provided.

Chrome increases the strength of steel. At a concentration of up to 0.08%, it does not adversely affect the weldability of sheets in pipe production, but expands the possibility of using scrap metal for smelting, which reduces the cost of steel.

The addition of copper, within the specified limits, increases the strength and corrosion resistance of steel. A higher copper content is not economically feasible.

Titanium is a strong carbide forming element that strengthens steel. Finely dispersed titanium carbides released during hot rolling and cooling of sheets with water are highly resistant to overheating. When the titanium content is less than 0.015%, the strength of the hot rolled sheets decreases. An increase in titanium content in excess of 0.030% leads to a decrease in the viscosity properties of the metal (in particular, at a temperature of -60 ° C), which is unacceptable for steels of this assortment.

Aluminum deoxidizes and modifies steel. By binding nitrogen to nitrides, it inhibits its negative effect on the properties of the sheets. When the aluminum content is less than 0.025%, the complex of mechanical properties of the sheets decreases. An increase in its concentration of more than 0.045% leads to a deterioration in the viscous properties of hot-rolled sheets.

The content of impurity elements of sulfur and phosphorus of the above limits leads to a deterioration in the plastic and viscous properties of hot-rolled sheets.

The limitation of the carbon equivalent value of less than 0.43 ensures high technology welding pipes at low ambient temperatures without preheating. The carbon equivalent is determined by the formula C _equiv = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the values of the elements correspond to their percentage in steel.

Heating a continuously cast slab to a temperature of 1200-1220 ° C and rough rolling at a temperature of 1040 ± 60 ° C for 7-10 passes with a compression ratio of at least 12% per pass to a thickness of 95 ± 15 mm allows the formation of static and dynamic recrystallization finely dispersed carbide phase, preventing the passage of collective recrystallization, and to ensure the grinding of the structure throughout the thickness.

Cooling the intermediate roll to a temperature of 860 ± 20 ° C and subsequent finishing rolling with a total degree of reduction of 75-85% and a temperature of the end of rolling of 855 ± 15 ° C provide dispersion hardening and grain refinement.

Accelerated cooling at a rate of 14-22 ° C / s to a temperature of 555 ± 15 ° C ensures the formation of the required phase composition over the entire cross section of the sheet. To stabilize the properties of plate steel and relieve residual internal stresses after accelerated cooling is completed, the sheets should be cooled more slowly in order to ensure the removal of residual internal stresses and normal processes occurring in the metal, which increases the level of mechanical properties of thick sheets. This approach contributes to obtaining a fine-grained equilibrium metal structure.

Thus, the full use of the resource of properties corresponding to low-alloy steel of a given chemical composition is ensured by the deformation-thermal regime of production of plate products. The technology of controlled rolling is aimed at obtaining the optimal phase composition and phase morphology, grinding of microstructure grains, hardening of solid solution and dispersion hardening.

An example implementation of the method.

The application of the method is illustrated by an example of its implementation in the production of a sheet of strength class K60. A blank was prepared containing, wt.%: C = 0.07; Si = 0.32; Mn = 1.67; S = 0.002; P = 0.009; Cr = 0.05; Ni = 0.26; Cu = 0.14; Al = 0.038; N = 0.005; V = 0.030; Ti = 0.019; Nb = 0.048, the rest is iron and impurities. In this case, the carbon equivalent is C _equiv = 0.43, i.e. corresponds to the declared range.

When preforms with a cross section of 300 × 2000 mm were heated to a temperature of 1215 ° С, dispersed carbonitride reinforcing particles were dissolved in low-alloy steel. After issuing from the furnace, rough rolling of the slab was carried out in the temperature range of 1020-1050 ° C to a thickness of 105 mm in 8 passes with a reduction of 12-14%. Then, the intermediate roll was cooled to a temperature of 855-870 ° C. Finishing rolling was performed in 9 passes with a total compression of 82.7%. Accelerated cooling of the roll with a thickness of 19 mm after leaving the stand of the plate mill was performed at a speed of 16-20 ° C / s to a temperature of 550-570 ° C. Then the sheets were edited with their slow cooling in air.

Mechanical properties were determined on transverse samples. The temperature-strain mode of rolling provided the production of a fine-grained ferrite-bainitic structure. Tensile tests were carried out on flat specimens in accordance with GOST 1497, and in impact bending on specimens with a V-shaped notch in accordance with GOST 9454 at a temperature of -20 ° C. The following mechanical properties were obtained for transverse samples (Table 2). The specified level of properties fully complies with the requirements for sheets of strength class K60.

The technical and economic advantages of the considered invention consist in the fact that the proposed temperature-deformation modes of production allow to use to the greatest extent all the mechanisms of hardening of low alloy steel of a given chemical composition: grinding of microstructure grains, dislocation hardening, dispersion hardening. Thus, the application of the proposed rolling method ensures the achievement of the desired result — the production of sheet metal for large diameter pipes with a level of mechanical properties corresponding to strength class K60 at a plate reversing mill.

Claims (1)

Method for the production of plate steel, including the production of a continuously cast slab, its heating, rough rolling, cooling the intermediate roll, its finish rolling, accelerated cooling of the finished roll to a predetermined temperature and subsequent delayed cooling, characterized in that the continuous cast slab is 300 ± thick 20 mm is obtained from steel containing in wt.%:
carbon 0.06-0.08 silicon 0.25-0.40 manganese 1,60-1,70 sulfur no more than 0,003 phosphorus no more than 0,013 chromium no more than 0.08 nickel 0.20-0.30 copper 0.10-0.20 aluminum 0.025-0.045 nitrogen no more than 0,008 vanadium 0,020-0,035 titanium 0.015-0.030 niobium 0,040-0,055 iron and impurities rest
moreover, the carbon equivalent is C _equiv ≤ 0.43, while the slab is heated to a temperature of 1200-1220 ° C, rough rolling is carried out at a temperature of 1040 ± 60 ° C for 7-10 passes with a reduction ratio of at least 12% per pass per thickness 95 ± 15 mm, the intermediate roll is cooled to a temperature of 860 ± 20 ° C, finish rolling is carried out with a total degree of compression of 75-85%, while the temperature of the end of rolling is set to 855 ± 15 ° C, and accelerated cooling of the finished rolling is performed at a speed of 14 -22 ° C / s to a temperature of 555 ± 15 ° C.