RU2218446C2 - Corrosion-resistant high-strength austenitic steel - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: invention relates to metallurgy, to compositions of corrosion- resistant high-strength austenitic steel and may be used at production of fittings, sheet and pipe-type details, fixture and other high-load details of the gas and oil producing equipment working at low temperatures in hostile environments, including environment with the high contents of hydrogen sulfide and carbon dioxide. The steel high performances are provided by its composition, that includes carbon, chromium, manganese, nickel, molybdenum, niobium, nitrogen, iron and additionally according to the invention, it contains boron and cerium all at the following shares (mass %): carbon 0.02-0.06, chromium 20.0-24.0, manganese 4.0-8.0, nickel 7.0-12.0, molybdenum 2.0-4.0, niobium 0.10- 0.30, nitrogen 0.40-0.70, boron 0.001-0.003, cerium 0.001-0.050, iron and imminent impurities - the rest, at fulfillment of the following ratio Σ(Cr+Mn+Mo) = 28.5-32.5. EFFECT: the invention allows to produce a corrosion-resistant high-strength austenitic steel with a high level of strength and viscous properties, durable operation at low temperatures and without hydrosulfuric cracking. 2cl, 4 tbl, 4 ex

Description

 The invention relates to the field of metallurgy, to compositions of corrosion-resistant high-strength austenitic steels and can be used in the manufacture of fasteners, sheet and pipe parts, fittings and other highly loaded parts of oil and gas production equipment operating at low temperatures in aggressive environments, including those with a high content of hydrogen sulfide and dioxide carbon.

Known steel resistant to hydrogen cracking, containing the following components, wt.%:
Carbon - 0.03-0.08
Silicon - 0.3-0.8
Manganese - 0.5-1.0
Chrome - 17.0-19.0
Nickel - 9.0-11.0
Titanium - 0.4-0.7
Molybdenum - 0.35-0.60
Cerium - 0.001-0.050
Nitrogen - 0.40-0.70
Iron - Else
(USSR copyright certificate 1038377, IPC C 22 C 38/50).

 Bars with a diameter of 30-90 mm and billets for producing a sheet of 1-3 mm were made from steel. Mechanical properties of steel: tensile strength 505-525 MPa, yield strength 205-225 MPa, relative elongation 41-46%. Steel is suitable for the manufacture of petrochemical and heat transfer equipment.

 The disadvantage of this steel is its low strength.

Known cast steel containing the following components, wt.%:
Carbon - 0.02-0.08
Manganese - 0.2-1.5
Silicon - 0.3-1.5
Chrome - 16.0-22.0
Nickel - 6.0-9.0
Molybdenum - 2.0-4.0
Copper - 1.0-2.5
Titanium - 0.05-0.20
REM - 0.01-0.10
Aluminum - 0.005-0.050
Calcium - 0.001-0.050
Niobium - 0.01-0.10
Iron - Else
while Σ (Al + Ti + Nb) ≤0.3 (USSR copyright certificate 1232701, IPC C 22 C 38/50).

Mechanical properties of steel: σ _b 830-850 MPa; σ _0.2 505-525 MPa; δ 20-24%; φ 42-47%. The resistance of steel to hydrogen sulfide cracking was determined by the exposure time of the test samples in a 3% NaCl solution, acidified with hydrogen sulfide to a concentration of 2550-3000 mg / l until brittle fracture. The exposure time was 910-1000 hours.

 Steel is designed for casting parts of fountain fittings and column head oil equipment operating in an aggressive environment at high pressures.

The strength level and resistance to sulfide stress cracking (SKRN) of the steel of this composition is insufficient for highly loaded parts (fasteners, rods, valves, ball valves, etc.) of oil and gas equipment operated at high pressures of up to 25 vol.% H ₂ S and up to 15 vol.% CO ₂ .

 The disadvantage is the instability of the austenitic base due to the possible formation of delta ferrite at a certain ratio of chromium, molybdenum and nickel.

Known corrosion-resistant steel containing the following components, wt. %:
Carbon - 0.04-0.08
Chrome - 23.0-27.0
Nickel - 3.5-5.0
Manganese - 3.5-6.0
Molybdenum - 2.5-3.5
Copper - 1.5-2.5
Silicon - 0.8-1.5
Nitrogen - 0.15-0.35
Niobium - 0.20-0.40
Zirconium - 0.05-0.15
Hafnium - 0.10-0.20
REM - 0.005-0.050
Lanthanum - 0.05-0.30
Iron - Else
(RF patent 2016133, IPC C 22 C 38/58).

Steel is designed for casting parts of equipment used to equip wells of oil and gas fields, including body parts of wellhead equipment. Mechanical properties of cast steel: σ _b 705-800 MPa; σ _0.2 460-630 MPa; δ 20-28%; φ 30-36%, KCU 5.5-10 J / cm ² .

 Steel resistant to embrittlement in an environment containing hydrogen sulfide and chlorine ions.

 The steel of this composition is not stably austenitic, but austenitic-ferritic, and depending on the ratio of austenitic and ferrite-forming elements within a given composition, the amount of ferritic component can reach 60-70%, which negatively affects the processability of steel and toughness at low temperatures.

Known austenitic corrosion-resistant high-strength steel containing the following elements, wt.%:
Carbon - ≤0.06
Chrome - 20.5-23.5
Nickel - 11.5-13.5
Manganese - 4.0-6.0
Molybdenum - 1.5-3.0
Silicon - ≤1.0
Niobium - 0.10-0.30
Nitrogen - 0.20-0.40
Sulfur - ≤0.03
Phosphorus - ≤0.04
Iron - Else
[Steel UNS S20910 (XM-19) ASTM A479 - prototype, ANNUAL BOOK OF ASTM STANDARDS, SECTION 1, Iron and Steel Products ASTM. Volume 01.03, Steel-Plate, Sheet, Strip, Wire, Stainless Steel Bar. p. 177-182].

Steel is used for the manufacture of parts for equipment for oil wells. Depending on the heat treatment and deformation modes, the mechanical properties of UNS S20910 steel change significantly. The highest level of strength characteristics (σ _0.2 ≥ 725 MPa according to ASTM A479) is achieved for bars in a cold-deformed state or after heat treatment and strain hardening. In the annealed state without special deformation modes, the strength of this steel is insufficient (σ _0.2 ≥ 380 MPa according to ASTM A479) due to the increased nickel content and lack of nitrogen.

 Since obtaining the required level of strength due to cold deformation is fraught with a number of technological difficulties and requires special equipment, the use of this method is very limited and not suitable for mass production of parts of a wide range.

The problem to which the invention is directed, is to create a corrosion-resistant steel, providing in the state of γ-solid solution without the use of special heat treatment and deformation modes, a high level of strength and viscous properties, including at temperatures up to -60 ^o C, high corrosion resistance and high resistance against hydrogen sulfide stress cracking.

The technical result of the invention is to obtain on a new steel a level of strength σ _of ≥ 860 MPa and σ _0.2 ≥ 725 MPa with a high level of ductility (δ≥20% for all types of rolled products, φ≥40% for rods and pipes) and viscosity (KV ^-60 ≥34 J / cm ² ) while providing resistance to hydrogen sulfide cracking under stress at the level of 0.8 of the minimum yield strength (725 MPa).

Сущность изобретения заключается в том, что в стали сбалансировано соотношение элементов, способствующих повышению растворимости азота в аустените, и элементов, увеличивающих сопротивление коррозии в контакте с сероводородом без образования второй фазы (δ-феррита).The specified technical result is achieved by the fact that the austenitic corrosion-resistant high-strength steel containing carbon, chromium, manganese, nickel, molybdenum, niobium, nitrogen and iron, according to the invention additionally contains boron and cerium in the following ratio of components, wt%:
Carbon - 0.02-0.06
Chrome - 20.0-24.0
Manganese - 4.0-8.0
Nickel - 7.0-12.0
Molybdenum - 2.0-4.0
Niobium - 0.10-0.30
Nitrogen - 0.40-0.70
Boron - 0.001-0.003
Cerium - 0.001-0.050
Iron and Impurities - Else
when the following relations
Σ (Cr + Mn + Mo) = 28.5 -32.5 and

The essence of the invention lies in the fact that in steel the ratio of elements contributing to an increase in the solubility of nitrogen in austenite and elements increasing the corrosion resistance in contact with hydrogen sulfide without the formation of a second phase (δ-ferrite) is balanced.

 The implementation of these qualities of steel occurs when the amount of nitrogen in this composition is 0.4-0.7%. The limitation of the upper limit on the nitrogen content is determined by the limit of its solubility during crystallization of steel of this composition. With a total content of ΣCr + Mn + Mo = 28.5-32.5, the most favorable conditions are achieved during smelting for the absorption of nitrogen in the liquid state.

 When the nitrogen content is less than 0.4%, the required level of strength is not achieved. The content of chromium in the amount of 21-24% and molybdenum in the amount of 2-4% provides high resistance to corrosion cracking in a hydrogen sulfide-containing medium and promotes the absorption of nitrogen in solid solution. The limitation of the upper limits on the content of chromium and molybdenum is associated with the need to prevent the formation of delta ferrite during high-temperature heating, which has a negative effect on the processability of steel during hot processing operations.

 When the chromium content is less than 20% and molybdenum 2%, the resistance of steel to the corrosion interaction of an aggressive environment is reduced. The limits for carbon content are established based on its effect in the steel of this composition on the process of sigma phase formation (reducing the tendency to form a sigma phase with increasing carbon concentration), as well as with the effect of carbon on strength. The upper limit on carbon content is limited due to lower viscosity at low temperatures. When the carbon content is less than 0.02%, the strength level is not realized. With a nickel content of 7-12%, a stable austenitic structure is obtained that is most resistant to hydrogen sulfide. The upper limit on the nickel content is limited to 12%, because with a larger quantity the solubility of nitrogen in the γ-solid solution decreases, which leads to increased gas porosity of the metal and a decrease in strength characteristics. When the nickel content is less than 7%, austenite becomes unstable, delta ferrite appears, which is undesirable.

 The role of niobium in the proposed steel is the formation of finely divided nitrides that perform the function of additional hardening. The amount of niobium must be strictly regulated so that the nitrogen limit can remain in the solid solution, and therefore the upper limit is limited to 0.3%. When the niobium content is less than 0.1%, the amount of the dispersed nitride phase is insufficient for hardening.

Указанная зависимость позволяет получить наилучшее сочетание твердорастворного и нитридного упрочнения.Boron and cerium in the proposed steel perform the function of preventing the formation of the nitride phase (niobium nitrides) along the grain boundaries. To ensure the highest nitrogen content in the solid solution and, accordingly, the strengthening effect of the content of boron, cerium, niobium and nitrogen, it is interconnected and must satisfy the following dependence:

The specified dependence allows you to get the best combination of solid solution and nitride hardening.

 Examples.

 The steel of the proposed composition was smelted in a 50 kg induction furnace and cast into molds for ingots weighing 25 kg. Table 1 presents the chemical composition of the proposed steel. The ingots were forged and rolled onto bars with a diameter of 16–20 mm.

 Tables 2-4 show the mechanical and corrosive properties.

The following methods were used as a criterion for the health of the proposed steel in a hydrogen sulfide-containing medium:
1. Tests for hydrogen sulfide cracking in accordance with the generally accepted method NACE 81-77 "A" (medium - 5% aqueous solution of NaCl and 0.5% acetic acid, saturated with H ₂ S at a pressure of 0.1 MPa, pH 0.3). Test temperature 23 ^o C. For the tests used cylindrical samples (d = 6 mm, f = 75 mm). Samples were tested under uniaxial tension with a constant load.

2. Corrosion cracking test in a model medium simulating high sulfur deposits (medium 5% aqueous solution of NaCl and 0.5% acetic acid saturated with H ₂ S and CO ₂ at a pressure of 1.5 MPa each at a temperature of 60 ^o C). For testing, flat samples measuring 3 • 10 • 75 mm were used.

 3. The axial tensile test of the samples after exposure to NACE "A" (1) for 720 hours.

 4. Test for general corrosion in a NACE environment (item 2) without applying a load for 360 hours.

The presented results of evaluating the mechanical and corrosion properties of the proposed steel indicate that the steel, having a high level of strength, has a fairly high corrosion resistance in hydrogen sulfide-containing media, which allows it to be used for the manufacture of highly loaded fittings and fasteners for oil and gas fields with a high degree of reliability during operation, including at temperatures up to -60 ^o C.

Claims (2)

1. Austenitic corrosion-resistant high-strength steel containing carbon, chromium, manganese, nickel, molybdenum, niobium, nitrogen and iron, characterized in that it additionally contains boron and cerium in the following ratio of components, wt.%: Carbon 0.02-0.06 Chrome 20.0-24.0 Manganese 4.0-8.0 Nickel 7.0-12.0 Molybdenum 2.0-4.0 Niobium 0.10-0.30 Nitrogen 0.40-0.70 Boron 0.001-0.003 Cerium 0.001-0.050 Iron and Inevitable Impurities when the relation Σ (Cr + Mn + Mo) = 28.5-32.5. 2. Steel according to claim 1, characterized in that the content of niobium, cerium, boron and nitrogen should be regulated by the ratio

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