RU2167954C2 - Structural steel - Google Patents

Abstract

FIELD: ferrous metallurgy, more particularly manufacture of structures serviceable at low temperatures, e.g. pipelines, sea stationary platforms, load-carrying structures of floating drilling rigs, reservoirs, building structures and machine components. SUBSTANCE: structural steel comprises, wt %: carbon, 0.08-0.22; silicon, 0.20-0.65; manganese, 1.25-2.20; titanium, 0.01-0.03; aluminium, 0.01-0.05; niobium, 0.01-0.23; boron, 0.001-0.005; zirconium, 0.01-0.05; nitrogen, 0.01-0.015; copper, 0.25-0.55; calcium 0.0015-0.055; yttrium, 0.0001-0.055; selenium, 0.00025-0.07; barium, 0.01-0.035; hafnium, 0.035-0.065; and iron, the balance. Steel comprises limited content of impurities, wt %: sulfur, up to 0.035 and phosphorus, up to 0.035. Higher resistance to fragile, destruction, higher plasticity with respect to thickness of rolled products, improved workability and weldability. EFFECT: improved properties of the structural steel. 2 cl, 1 ex, 2 tbl

Description

 The invention relates to ferrous metallurgy, in particular to alloys of the steel group, and can be used for the manufacture of structures operating at low temperatures, for example, for pipelines, offshore stationary platforms, supporting structures of drilling floating units, tanks, building structures, machine parts.

 Known steel (A. S. USSR 1052558 A; C 22 C 38/14, 1982) containing, by weight. %: carbon - 0.08-0.20; silicon - 0.2-0.6; Manganese - 1.2-2.0; titanium - 0.01-0.10; aluminum - 0.01-0.06; boron - 0.0005-0.005; yttrium - 0.0001-0.05; nitrogen - 0.005-0.025; calcium - 0.0002-0.06; iron is the rest.

As impurities, the steel may contain, wt.%:
Sulfur - up to 0.035
Phosphorus - up to 0.035.

 A disadvantage of the known steel is its low viscosity at low temperatures. It has poor machinability, weldability and high impact strength anisotropy.

 The closest in composition to the technical nature and the achieved result is steel, described in the USSR copyright certificate N 1100332 A, according to class. C 22 C 38/60, 1982 (prototype) containing, wt.%: Carbon - 0.08-0.20; silicon -0.20-0.60; manganese -1.2-2.0; titanium - 0.01-0.10; aluminum - 0.01-0.06; boron - 0.0005-0.004; barium - 0.001-0.03; selenium - 0.0002-0.08 yttrium -0.0001-0.05; nitrogen - 0.005-0.025; iron is the rest.

As impurities, the steel may contain, wt.%:
Sulfur - up to 0.035
Phosphorus - up to 0.035.

Known steel after hardening and high tempering has the following set of mechanical properties:
σ _in = 550-600 N / mm ² ,
σ _t = 460-480 N / mm ² ,
δ ₅ = 17-22%
KCU ⁺²⁰ ≥ 220 J / cm ² ;
KCV ^-40 > 100 J / cm ² ;
% B (-40 ^o C) ≈ 50.

A disadvantage of the known steel is low strength, ductility (φ _z ) in the z direction, significant anisotropy of the viscous characteristics, which determines the low resistance of the steel to brittle fracture.

 The main task, which the proposed steel is aimed at, is to increase resistance to brittle fracture, increase ductility in the direction of the thickness of the rolled product (in the z direction), and improve machinability and weldability.

 The above technical result is achieved in that in the known structural steel containing carbon, silicon, manganese, titanium, aluminum, boron, yttrium, barium, selenium, nitrogen, sulfur, phosphorus and iron, zirconium, copper, calcium, niobium, hafnium are additionally introduced in the following ratio of components, wt. %: carbon - 0.08-0.22; silicon - 0.20-0.65; Manganese - 1.25-2.20; titanium - 0.01-0.03; aluminum - 0.01-0.05; niobium - 0.01-0.03; boron - 0.001-0.005; zirconium - 0.01-0.05; nitrogen - 0.01-0.025; copper - 0.25-0.55; calcium - 0.0015-0.055; yttrium - 0.0001-0.055; selenium - 0.00025-0.07; barium - 0.01-0.035; hafnium - 0.035-0.065; iron is the rest.

Impurities:
Sulfur - up to 0.035
Phosphorus - up to 0.035
Iron is the rest.

 The proposed steel at high values of strength, ductility (especially in the z direction), cold resistance, also has high technological properties.

The proposed steel has the following values of mechanical properties after hardening and high tempering:
Tensile strength,
σ _in , H / mm ² - 650-690
Yield strength
σ _t , H / mm ² - 520-580
Relative extension,
δ ₅ ,% - 22-29
Relative narrowing
φ,% - 55-65
Impact strength
KCU ⁺²⁰ , J / cm ² - 240-280
KCV ^-40 - 150-175
% B (-40 ^o C) ≈ 50-70
Anisotropy coefficient of viscous characteristics
KCUprod. / KCUper. - 1.4 - 1.8
As you know, the simultaneous increase in strength and cold resistance is a difficult task, since the stronger the steel, the lower the resistance to brittle fracture. This problem can be solved by obtaining ultrafine grains. Grain grinding is achieved by microalloying with carbonitride-forming elements (niobium, vanadium, titanium, zirconium, etc.). In the proposed steel, the production of ultrafine grains is achieved by the introduction of hafnium, yttrium, which in combination with titanium, zirconium and boron provides 12-13 points of grain, which is impossible with microalloying with titanium, zirconium or boron. In addition, hafnium, yttrium remove all nitrogen from the solid solution, which further increases the cold resistance, and being a part of complex corbonitrides, significantly increases their resistance and dispersion. The most optimal combination of strength, ductility in the z - direction, cold resistance and weldability is achieved with the experimentally found sum of titanium, niobium, zirconium, hafnium and yttrium in the range of 0.95-0.2%.

 An increase in technological properties is usually achieved by the introduction of calcium and an increase in sulfur content. In this case, an increase in the content of sulfur and calcium is excluded, since in this case the cold resistance decreases sharply. In the proposed steel, to solve this problem, calcium, hafnium and yttrium are simultaneously introduced, which, without reducing the cold resistance characteristics, significantly increase workability, improve weldability and strength.

 The carbon limits are limited to 0.08-0.22 wt.%. The carbon content below 0.08 does not provide sufficient strength and fluidity when the content of other elements at the lower and middle levels. The upper carbon limit is limited to 0.22 wt. % The carbon content above this limit reduces the cold resistance of steel with the content of the remaining elements at the middle and upper levels.

 The lower limit of the silicon content is determined to be 0.20 wt.%, Below which the metal does not have sufficient deoxidation. The upper limit of the silicon content is limited to 0.65 wt.%, Above which the brittle fracture resistance of the steel decreases.

 The limits for manganese are selected in the range of 1.25-2.2. wt.%. A manganese content below 1.25 wt.% Does not provide sufficient deoxidation of the metal and sufficient strength properties when the content of the remaining elements is at the middle and lower levels. The upper limit for manganese is determined by 2.2 wt.%. Above this limit, the formation of carbonitrides slows down and the cold resistance of steel decreases.

 The titanium content is limited to 0.01-0.03 wt.%. Below this limit, the steel is not sufficiently deoxidized and the hardness of complex carbonitrides is not ensured. The upper titanium content is limited to 0.03 wt.%. Above this limit, embrittlement of steel occurs and the processability of steel deteriorates.

 The limits for aluminum are limited to 0.01-0.05 wt.%. When the aluminum content is below 0.01%, the metal is not sufficiently deoxidized. The upper aluminum content is limited to 0.05 wt.%, Above this limit, the processability and surface quality of the rolled products are deteriorated.

 The lower limit for boron is 0.001 wt.%. When the boron content is below this limit, the necessary hardenability and workability are not provided. The upper limit is chosen to be 0.005 wt.%, The boron content above this limit causes the release of excess borides along the grain boundaries, which sharply reduces the resistance of steel to brittle fracture.

 The niobium content is selected in the range of 0.01-0.03 wt.%. The content of niobium below 0.01 wt.% Does not provide the formation of a sufficient amount of carbonitrides, which does not affect the delay in grain growth. When the niobium content is above 0.03 wt. % an excess amount of carbonitrides appears, which can reduce the cold resistance.

 Zirconium is introduced in the range of 0.01-0.05 wt.%. A zirconium content below 0.01 wt.% Does not provide the required level of strength. The content of zirconium above 0.05 wt.% Causes the formation of an excessive amount of carbonitrides, which leads to a decrease in the resistance of steel to brittle fracture.

 The nitrogen limits are limited to 0.01-0.025 wt.%. When the nitrogen content of 0.010 wt. % the possibility of the formation of carbonitrides is not provided, the nitrogen content above 0.025 wt.% causes a decrease in cold resistance due to the appearance of free nitrogen in the solid solution.

 The limits for copper are in the range of 0.25-0.55 wt.%. The copper content is below 0.25 wt. % is not enough for the formation of the ε phase, above 0.55 wt.% can cause red breaking.

 The lower limit for calcium is 0.0015 wt.%. The calcium content below this limit does not provide globularization of sulfides, above 0.055 wt. % causes steel pollution by an excessive amount of non-metallic inclusions, which will negatively affect the cold resistance.

 The lower yttrium content is 0.0001 wt.%. This is the beginning of the positive effect of this element on workability and viscosity. The upper yttrium content in steel is limited to 0.055 wt.%. Above this amount of yttrium, the steel is substantially embrittled.

 The lower limit of hafnium content is selected to 0.035 wt.%. The content of hafnium below this limit does not provide ultrafine grains and, accordingly, the optimal combination of strength and cold resistance. The upper limit of hafnium content is limited to 0.065 wt.%. At hafnium contents above this limit, steel is not technologically advanced.

 The lower barium limit of 0.01 wt.% Determines the beginning of its beneficial effect on workability and viscosity. The upper barium content is 0.035 wt.%. Above this amount, steel is embrittled.

 The lower selenium content is 0.00025 wt.%, Determines the beginning of its positive effect on workability and viscosity due to globularization of inclusions. The upper selenium content is 0.07 wt.%. Above this amount, selenium embrittles steel.

 The presence of the indicated amounts of barium, selenium and hafnium in the proposed steel promotes globularization and a change in the composition of non-metallic inclusions, and also reduces the anisotropy of the viscous characteristics, thereby increasing the steel's resistance to brittle fracture.

 The following are embodiments of the invention, not excluding other options within the scope of the claims.

 Experienced steels were smelted in an induction 50 kg furnace on Armco iron. Preliminary deoxidation was carried out with silico-calcium and ferromanganese, and final - with aluminum. The chemical compositions of steel with the corresponding content of elements are presented in table. 1. Melting 6 - known steel, taken as a prototype.

 Ingots (25 kg) were forged into 45 x 500 mm casing, and then rolled onto a sheet with a thickness of 15 - 40 mm. There were no technological difficulties during forging and rolling.

 Tensile properties were determined according to GOST 1497-84 on explosive samples KR-1, serial impact tests on specimens "U", "V" and "T" were determined according to GOST 9454-79.

Machinability was determined in the annealed condition on the workpieces for conditions of semi-turning without cooling for pure metal with cutters equipped with hard alloys at constant values of the cutting depth of 1.5 mm, feed 0.2 mm / rev and the main angle in terms of cutters 60 ^o . Machinability is estimated by the cutting speed corresponding to the 60-minute resistance of the cutters B ₆₀ and is expressed by the coefficient K _TV.spl. in relation to reference steel 45, the cutting speed of which V _{60 is} taken per unit.

 The properties of steel are given in table. 2. As can be seen from the obtained data, the proposed steel at a sufficiently high level of strength and plastic properties has an increased resistance to viscous fracture.

 Thus, the proposed composition of structural steel in comparison with the prototype has high strength, ductility (especially in the z direction), cold resistance, also has high technological properties, which are ensured by the additional introduction of the following elements: zirconium, copper, calcium, yttrium , niobium, hafnium.

 The use of the proposed steel, for example, for the manufacture of offshore stationary platforms, supporting structures of floating drilling rigs and trunk pipelines will significantly reduce the metal consumption of the structures, and will also increase reliability by 2.0-2.5 times due to the increase in cold resistance.

Claims (1)

1. Structural steel containing carbon, silicon, manganese, titanium, aluminum, boron, nitrogen, yttrium, selenium, barium and iron, characterized in that it additionally contains zirconium, copper, calcium, niobium and hafnium in the following ratio of components, wt .%:
Carbon - 0.08 - 0.22
Silicon - 0.20 - 0.65
Manganese - 1.25 - 2.20
Titanium - 0.01 - 0.03
Aluminum - 0.01 - 0.05
Niobium - 0.01 - 0.03
Boron - 0.001 - 0.005
Zirconium - 0.01 - 0.05
Nitrogen - 0.01 - 0.025
Copper - 0.25 - 0.55
Calcium - 0.0015 - 0.055
Yttrium - 0.0001 - 0.055
Selenium - 0.00025 - 0.07
Barium - 0.01 - 0.035
Hafnium - 0.035 - 0.065
Iron - Else
2. Steel according to claim 1, characterized in that it further limits the content of impurities, wt.%:
Sulfur - Up to 0.035
Phosphorus - Up to 0.035

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