RU2180016C1 - Steel for oil-trunk and gas-main pipelines - Google Patents

Abstract

FIELD: steelmaking. SUBSTANCE: invention relates to low-alloy steels resistant against hydrogen cracking for use in manufacturing welded oil- and gas-pipeline pipes suitable for exploiting under far North conditions. Steel contains, wt %: carbon 0.04-0.12, manganese 0.7-1.7, silicon 0.2-0.9, vanadium 0.03-0.12, niobium 0.02-0.08, aluminum 0.02- 0.06, nitrogen 0.004-0,01-, calcium 0.001-0.02, chromium up to 0.3, nickel up to 0.3, copper up to 0.3, titanium up to 0.03, sulfur up to 0.008, phosphorus up to 0.0015, molybdenum 0.001-0.15, and iron - the balance. EFFECT: increased viscosity and improved workability of steel. 3 tbl

Description

 The invention relates to metallurgy, in particular to low alloy steels resistant to hydrogen cracking, used for the manufacture of welded oil and gas pipelines suitable for operation in the Far North.

 Low alloy steel for oil and gas pipelines operating in the Far North should combine high strength, weldability, corrosion resistance and resistance to hydrogen cracking in aggressive sulfide-containing environments (Table 1).

Known low-alloy steel for oil and gas pipelines [1], having the following chemical composition, wt.%:
Carbon - 0.20 - 0.40
Silicon - 0.10 - 0.40
Manganese - 0.50 - 1.2
Phosphorus - ≤ 0.010
Sulfur - ≤ 0.003
Chrome - 0.080 - 1.50
Molybdenum - 0.10 - 0.60
Titanium - 0.005 - 0.03
Niobium - 0.005 - 0.10
Aluminum - 0.01 - 0.06
Calcium - 0.001 - 0.005
Nitrogen - 0.0010 - 0.0090
Iron - Else
Steel of known composition has low viscosity properties at test temperatures of -60 ^° C and insufficient resistance to hydrogen cracking.

Also known low alloy steel [2] the following chemical composition, wt.%:
Carbon - 0.05 - 0.15
Manganese - 1.2 - 2.0
Silicon - 0.15 - 0.60
Vanadium - 0.03 - 0.15
Niobium - 0.005 - 0.10
Aluminum - 0.006 - 0.06
Nitrogen - 0.002 - 0.015
Titanium - 0.005 - 0.10
Chrome - 0.01 - 0.30
Nickel - 0.01 - 0.30
Copper - 0.01 - 0.30
REM - 0.002 - 0.050
Sulfur - ≤ 0.01
Phosphorus - ≤ 0.02
Iron - Else
This steel is also characterized by insufficient viscosity at low temperatures and has low resistance in environments containing sulfide compounds. All this does not allow the use of well-known steel for trunk oil and gas pipelines operating in the Far North.

The closest in its chemical composition and properties to the proposed steel is steel [3], adopted as a prototype and containing, wt.%:
Carbon - 0.03 - 0.11
Manganese - 0.90 - 1.80
Silicon - 0.06 - 0.60
Chrome - 0.005 - 0.30
Nickel - 0.005 - 0.30
Vanadium - 0.02 - 0.12
Niobium - 0.03 - 0.10
Titanium - 0.010 - 0.040
Aluminum - 0.010 - 0.055
Calcium - 0.001 - 0.005
Sulfur - 0.0005 - 0.008
Phosphorus - 0.0005 - 0.010
Nitrogen - 0.001 - 0.012
Copper - 0.005 - 0.25
Antimony - 0.0001 - 0.005
Tin - 0.0001 - 0.007
Arsenic - 0.0001 - 0.008
Iron - Else
moreover, the total content of phosphorus P, antimony Sb, arsenic As, and tin Sn should satisfy the ratio: 2P + Sn + Sb + As <0.035.

 The disadvantages of steel of known composition are that it is characterized by low viscosity properties, especially at low temperatures, insufficient corrosion resistance (does not withstand tests for resistance to hydrogen cracking). Steel is not technologically advanced in production, since it requires special measures to remove phosphorus, the introduction of antimony, tin and arsenic in regulated quantities. In addition, antimony, tin and arsenic significantly impair the mechanical properties of steel, as a result of which their introduction as alloying elements does not seem appropriate.

 The technical problem solved by the invention is to increase the viscosity properties, resistance to hydrogen cracking and manufacturability of steel production.

To solve the technical problem, steel containing carbon, manganese, silicon, vanadium, niobium, aluminum, nitrogen, calcium, chromium, nickel, copper, titanium, sulfur, phosphorus and iron, additionally contains molybdenum in the following ratio, wt.%:
Carbon - 0.04 - 0.12
Manganese - 0.7 - 1.7
Silicon - 0.2 - 0.9
Vanadium - 0.03 - 0.12
Niobium - 0.02 - 0.08
Aluminum - 0.02 - 0.06
Nitrogen - 0.004 - 0.010
Calcium - 0.001 - 0.020
Chrome - ≤ 0.3
Nickel - ≤ 0.3
Copper - ≤ 0.30
Titanium - ≤ 0.03
Sulfur - ≤ 0.008
Phosphorus - ≤ 0.015
Molybdenum - 0.001 - 0.15
Iron - Else
Carbon in the steel of the proposed composition determines its strength properties. A decrease in carbon content of less than 0.04% leads to a drop in strength below an acceptable level. An increase in carbon content in excess of 0.12% impairs the ductility and toughness of the steel.

 Manganese is introduced to deoxidize and increase the strength of steel, binding impurity sulfur to sulfides. When the manganese content is less than 0.7%, the strength of steel and viscosity at low temperatures are reduced. An increase in the concentration of manganese in excess of 1.7% leads to the formation of a bainitic structure in the middle of the thickness of the rolled product, which reduces cold resistance and worsens the resistance against hydrogen cracking.

 Silicon deoxidizes and strengthens the steel, increases its elastic properties, and strengthens the ferrite phase. When the silicon content is less than 0.2%, the strength of the steel is insufficient. An increase in the silicon content of more than 0.9% leads to an increase in the number of silicate non-metallic inclusions, embrittlement of steel, and worsens its ductility.

 Vanadium and niobium form carbides VC, NbC with carbon, and nitrides VN, NbN with nitrogen. Small nitrides and carbonitrides of vanadium and niobium are located along the boundaries of grains and subgrains, inhibit the movement of dislocations and thereby strengthen steel. When the content of vanadium is less than 0.03% and niobium is less than 0.02%, their effect is not large enough, the properties of steel are below the permissible level. An increase in the concentration of vanadium over 0.12% or niobium over 0.08% causes precipitation hardening and leads to embrittlement of grain boundaries. This degrades the properties of steel.

 Aluminum is a deoxidizing and modifying element. When the aluminum content is less than 0.02%, its effect is weak, steel has low mechanical properties. An increase in aluminum content of more than 0.06% leads to a deterioration in weldability.

 Nitrogen is a carbonitride forming element that strengthens steel. Therefore, with a decrease in the nitrogen concentration of less than 0.004%, the strength properties of steel become lower than the permissible level. An increase in nitrogen concentration in excess of 0.010% leads to a decrease in the viscosity properties at low temperatures, which is unacceptable.

 Calcium is a modifying element. In addition, it binds sulfur to globular sulfides, increasing the viscosity properties of steel. At a calcium concentration of less than 0.001%, its effect is weak. An increase in calcium concentration of more than 0.02% increases the number and size of non-metallic inclusions, worsens the resistance to hydrogen cracking.

 Chrome, nickel and copper contribute to the increase of strength properties, but when the content of each of these elements is more than 0.3%, there is a decrease in the toughness of steel at low temperatures.

 Titanium is a strong carbide forming element that strengthens steel. However, during welding, titanium completely burns out, so its amount in steel should not exceed 0.03%.

 Sulfur and phosphorus are harmful impurities that reduce the plastic and viscous properties. At a sulfur concentration of not more than 0.008% and phosphorus not more than 0.015%, their harmful effect is weak and does not lead to a noticeable decrease in the mechanical properties of steel. At the same time, a deeper removal of sulfur and phosphorus makes steel more expensive and makes its production non-technological.

 Molybdenum in an amount of 0.001-0.15% provides an increase in the resistance of this steel against hydrogen cracking, increases the viscosity at low temperatures. At a molybdenum concentration of less than 0.001%, the steel does not pass the test for resistance to hydrogen cracking and, in the hot-rolled state, the strips are prone to embrittlement. An increase in the concentration of molybdenum in excess of 0.15% does not lead to a further improvement in its mechanical properties, but only increases the cost of alloying materials.

 Table 2 shows the chemical compositions of steels with different contents of alloying elements and impurities, and in table 3 the properties of these steels.

 As follows from the table. 2 and 3, steel of the proposed composition (compositions 2-4) has higher viscosity properties at low temperatures, as well as increased resistance to hydrogen cracking. By eliminating the need for deep desulfurization and dephosphorization, the manufacturability of its production is increased.

 In cases of transcendental values of the concentration of alloying elements and impurities (compositions 1 and 5), as well as when using steel of known chemical composition (composition 6), adopted as a prototype, the viscosity properties of steel deteriorate. The steels of these compounds do not stand the test for resistance to hydrogen cracking and are not technologically advanced in production.

Claims (1)

Steel for oil and gas pipelines containing carbon, manganese, silicon, vanadium, niobium, aluminum, nitrogen, calcium, chromium, nickel, copper, titanium, sulfur, phosphorus and iron, characterized in that it additionally contains molybdenum in the following ratio components, wt. %:
Carbon - 0.04-0.12
Manganese - 0.7-1.7
Silicon - 0.2-0.9
Vanadium - 0.03-0.12
Niobium - 0.02-0.08
Aluminum - 0.02-0.06
Nitrogen - 0.004-0.010
Calcium - 0.001-0.02
Chrome - ≤ 0.3
Nickel - ≤ 0.3
Copper - ≤ 0.3
Titanium - ≤ 0.03
Sulfur - ≤ 0.008
Phosphorus - ≤ 0.015
Molybdenum - 0.001-0.15
Iron - Else

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