RU2790717C1 - Unstabilized austenitic steel resistant to local corrosion in scp-water - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to unstabilised high-strength corrosion-resistant austenitic steels used in the manufacture of elements of nuclear power units operating under supercritical pressure (SCP) coolant in contact with water at temperatures up to 580°C. The steel contains, wt.%: carbon not more than 0.04, silicon 1.2-1.9, manganese from more than 3.0 to 5.0, chromium from more than 25.0 to 28.0, nickel 20.0-23.0, molybdenum 2.0-2.5, nitrogen 0.10-0.21, vanadium 0.02-0.12, tungsten 0.10-0.20, boron 0.0005-0.008, calcium 0.001-0.015, cerium 0.001-0.005, iron and the rest is inevitable impurities. For steel components, the following ratio holds: (1.5[Cr]+1.2[Mn]+1.1[Mo]+2.4[V]+0.5[W]-1.7[Si]-1.2[Ni]-2.3[B])/([N]+[C])≥90 and ([Cr]+[Mo]+1.5[Si]+0.5[W]+[V])/([Ni]+0.5[Mn]+30[N]+30[C ])≤ 1.10. After heating for 2000 hours at 580°C the thermal stability of steel is provided, its resistance to sensitization and sigmatization of the structure, to intergranular and pitting corrosion, cracking in a chloride-containing environment, while increasing strength and maintaining ductility.

EFFECT: thermal stability of steel.

1 cl, 2 tbl

Description

The invention relates to metallurgy, namely to high-strength corrosion-resistant austenitic steels used in the manufacture of elements of nuclear power units operating under supercritical pressure (SCP) coolant in contact with water at temperatures up to 580°C. Known corrosion-resistant steel containing, wt. %:

carbon 0.01-0.10 silicon 0.05-2.0 manganese 0.1-3.0 chromium 17.0-26.0 nickel 11.0-24.5 molybdenum 1.0-5.0 nitrogen 0.05-0.40 vanadium 0.01-0.25 cerium 0.01-0.05 calcium 0.001-0.150 iron and inevitable impurities rest,

when the following relations are fulfilled:

(%V+%Ce+%Ca)/(%C+%N)=0.25-0.65,

0.5<(%Ni+0.5%Mn)/(%Cr+%Mo+1.5%Si+%V)<0.90,

%Ni+16(%C+%N)-(%Cr+l,5%Mo-20) ² /12=14-24,

while sulfides in steel do not exceed 2 points, and line and point nitrides and carbonitrides - no more than 3 points for each type.

(RF patent No. 2409697, IPC C22C 38/58, C22C 38/46, published on January 20, 2011).

The disadvantages of the known steel are low resistance to long-term thermal aging at 580°C, susceptibility to intergranular corrosion (ICC) and intergranular corrosion cracking in chloride-containing environments. Known corrosion-resistant alloy 625, containing, wt. %:

carbon ≤0.10 silicon ≤0.50 manganese ≤0.50 chromium 20.0-23.0 nickel ≥58.0 molybdenum 8.0-10.0 niobium 3.15-4.15 iron ≤5.0 cobalt ≤1.0 titanium ≤0.40 aluminum ≤0.40 phosphorus ≤0.015 sulfur ≤0.015

(Pyshin I.V., Belov I.A., Sedov A.A. et al. Federal State Unitary Enterprise RRC "Kurchatov Institute". Problems of corrosion and mass transfer in the reactor circuit of a power vessel reactor with supercritical water parameters. Proceedings of the 7th MNTK "Ensuring the safety of nuclear power plants with VVER", May 17-20, 2011 OKB "Gidropress", Podolsk, Russia.)

The disadvantages of the known alloy are the high nickel content and, as a consequence, the low solubility of interstitial elements (carbon and nitrogen) in austenite. causing a drop in the thermal stability of the metal and the release of secondary phases during prolonged heating at temperatures of 500 ° C and above, initiating the tendency of the alloy to pitting corrosion.

The closest to the proposed invention in terms of essential features and the achieved result is a corrosion-resistant austenitic steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, vanadium, tungsten, niobium, boron, iron and inevitable impurities in the following ratio, wt . %:

carbon 0.05-0.04 silicon 1.6-3.0 manganese 0.5-2.0 chromium 18.2-21.0 nickel 13.0-18.0 molybdenum 1.8-3.8 nitrogen 0.1-0.4 vanadium 0.01-0.5 tungsten 0.1-1.0 niobium 0.01-0.5 boron 0.0005-0.008 iron and inevitable impurities rest,

when the ratio is fulfilled:

([Cr]+[Mo]+l.5[Si]+0.5[Nb+W])/([Ni]+30[C+N]+0.5[Mn])=0.95- 1.25

where [Cr]+[Mo]+l.5[Si]+0.5[Nb+W]≤28.5 and

[Ni]+30[C+N]+0.5[Mn]≤27.0

and when doing the following dependency:

[Cr]+3.3[Mo+Si]+16[N]+0.5W≥35.

(Patent of the Russian Federation No. 2499075, IPC C22C 38/58, C22C 38/54, pub. 20.11.213).

The disadvantages of the known steel are low resistance to thermal aging, precipitation of secondary phases during prolonged heating at 580°C, leading to steel sensitization and, as a result, to its intergranular, pitting corrosion and stress corrosion cracking in chloride-containing media. When heated to 500°C and higher after welding, niobium-stabilized steel becomes prone to knife corrosion and local destruction of the heat-affected zone.

In supercritical coolant reactors, water is in a liquid state, in which the electrochemical mechanism of metal corrosion is realized. Therefore, the claimed steel, unlike the prototype, must be thermally stable. In the process of prolonged heating, no precipitation of secondary phases should occur in its structure, causing the metal to be prone to local corrosion.

The problem solved by the proposed application for the invention is to create unstabilized stainless high-strength austenitic steel for elements of nuclear power plants SKD, resistant to local types of corrosion under conditions of prolonged contact with water at temperatures up to 580°C.

The technical result of the invention is to increase the resistance against thermal aging of steel (sensitization - precipitation of secondary phases along the grain boundaries during prolonged heating), to eliminate the tendency to intergranular, pitting corrosion and corrosion cracking after prolonged heating at temperatures up to 580 ° C. As a result of the exclusion of niobium from the chemical composition of steel, the tendency of the metal to knife corrosion and local destruction of the near-weld zone is eliminated.

The technical result of the invention is achieved in that the unstabilized corrosion-resistant austenitic steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, vanadium, tungsten, boron, according to the invention, additionally contains calcium and cerium in the following ratio, wt. %:

carbon no more than 0.04 silicon 1.2-1.9 manganese from more than 3.0 to 5.0 chromium from more than 25.0 to 28.0 nickel 20.0-23.0 molybdenum 2.0-2.5 nitrogen 0.10-0.21 vanadium 0.02-0.12 tungsten 0.10-0.20 boron 0.0005-0.008 calcium 0.001-0.015 cerium 0.001-0.005 iron and inevitable impurities rest,

when the ratios are fulfilled:

ECTS=(1.5[Cr]+1.2[Mn]+1.1[Mo]+2.4[V]+0.5[W]-1.7[Si]-1.2[Ni ]-2.3[B])/([N]+[C])≥90,

ESPS=([Cr]+[Mo]+1.5[Si]+0.5[W]+[V]) / ([Ni]+0.5[Mn]+30[N]+30[C ])≤1.10,

where ECTS is the equivalent of structural thermal stability or resistance of steel against thermal aging;

ESPS is the equivalent of steel resistance against sigmatization.

The limits of the content of alloying elements are determined based on the results of tests of steel of different chemical compositions and on the basis of the structural diagrams of Sheffler, Potak-Sagalevich, the Fe-Cr-Ni ternary system, taking into account the role of individual components in the steel structure formation.

The upper limit on carbon content is limited to 0.04% in order to prevent the tendency of steel to intergranular corrosion in chloride-containing environments.

The content of manganese in the range from more than 3.0 to 5.0% is determined by its amount necessary for sufficient assimilation of nitrogen in order to prevent the release of secondary phases (intermetallic compounds such as FeCr, nitrides and carbonitrides) during prolonged heating of steel. At a higher concentration of manganese, the resistance of steel to local corrosion decreases due to the increased depassivating effect of the chloride ion on the metal surface.

The established limits of chromium content from more than 25.0 to 28.0% provide the thermal stability of the steel structure during its long-term heating and the necessary protection against local corrosion in SKD-water at temperatures up to 580°C.

The limitation of the upper limit - 28.0% is associated with the need to prevent the formation of an intermetallic sigma phase in the steel structure, which has a negative effect on the corrosion resistance, mechanical and technological properties of the metal. When the chromium content is less than 25.0%, the steel becomes prone to thermal aging at 580°C and the release of secondary phases, initiating a drop in the resistance of steel against local types of corrosion.

The nickel content in the range of 20.0-23.0% is due to the need to prevent the formation of delta ferrite in the steel structure during its prolonged heating. At a lower concentration of nickel, its efficiency decreases. A higher nickel content will lead to an unjustified increase in the cost of steel and a decrease in its resistance to thermal aging.

Molybdenum and silicon in the range of 2.0-2.5 and 1.2-1.9%, respectively, ensure the resistance of steel against intergranular and pitting corrosion in water at 580°C. When the content of molybdenum and silicon is less than the lower limits, the resistance of steel to local corrosion decreases. The concentration of molybdenum and silicon, exceeding the upper limits, causes the formation of delta ferrite and silicates in the structure of inclusions, respectively, which adversely affect the manufacturability of steel during its hot deformation.

Alloying with nitrogen in the range of 0.10-0.21% is necessary to ensure high strength and prevent pitting corrosion of steel during its long-term heating in SKD water. When the nitrogen content is less than 0.10%, the required level of these properties is not realized. A nitrogen concentration of more than 0.21% can, during operation at a temperature of 580°C, cause the formation of chromium nitrides along the boundaries of austenite grains, initiating local corrosion of the steel.

Vanadium in nitrogen-containing steel limits the growth of austenite grains, forms clusters of finely dispersed nanosized nitrides, which perform the function of cathodic protection against corrosion in water and additional (to solid-solution) hardening of the metal. The upper limit on the content of vanadium is limited to 0.12% so that the maximum amount of nitrogen can remain in solid solution. When the content of vanadium is less than 0.02%, the effectiveness of its influence on the properties of steel falls.

Alloying steel with tungsten enhances its strength characteristics at a water temperature of 580°C and supercritical pressure. The tungsten content of more than 0.2% contributes to the formation of delta ferrite in the metal structure, which reduces the resistance of manganese steel to corrosion cracking in water. When the concentration of tungsten is less than 0.10%, its effect on the strength properties is reduced.

Alloying with boron helps to clean grain boundaries from dispersed inclusions, increase the technological plasticity of steel and its resistance to intergranular corrosion. With a boron content of more than 0.008%, excess phases are formed along the austenite grain boundaries - borides, which reduce the technological properties and corrosion resistance of steel. Boron concentration less than 0.0005% is ineffective.

Cerium (0.001-0.005%) and calcium (0.001-0.015) in the proposed steel perform the function of preventing the formation of nitrides and carbonitrides along the austenite grain boundaries.

Calcium increases the toughness of steel due to the effect of cleaning from grain-boundary impurities (sulfur, phosphorus, non-ferrous metals) and increases the technological plasticity of nitrogen-containing silicon steel during its hot and cold deformation. With an increase in the calcium content of more than 0.015%, the contamination of steel with non-metallic inclusions may increase. In an amount of less than 0.001%, calcium and cerium practically do not affect the properties of steel, so their content is not effective. The cerium content above 0.005% for this steel is not economically feasible.

Ratio

ECTS=(1.5[Cr]+1.2[Mn]+1.1[Mo]+2.4[V]+0.5[W]-1.7[Si]-1.2[Ni ]-2.3[B])/([N]+[C])≥90 within the stated limits ensures the production of unstabilized austenitic nitrogen-containing silicon steel, not prone to long-term thermal aging (separation of secondary phases in austenite) at a temperature of 580°C and, as a result, resistant to local types of corrosion. At ECTS values below 90, the resistance of steel against thermal aging decreases. As a result, during prolonged heating of the metal, secondary nitride, carbide, and carbonitride phases precipitate in its structure.

To prevent sigmatization of the inventive steel during its long-term heating at 580°C, the following ratio of ferrite- and austenite-forming elements must be fulfilled:

ESPS=([Cr]+[Mo]+1.5[Si]+0.5[W]+[V]) / ([Ni]+0.5[Mn]+30[N]+30[C ]) ≤ 1.10

When ESSP is more than 1.10, during heating, the sigma phase is released in steel, initiating its tendency to local types of corrosion in SKD water.

The essence and examples of the invention are illustrated in tables 1 and 2, which presents the chemical compositions and test results of the claimed steel and prototype.

Steel is smelted in an open induction furnace and poured into ingots weighing 17 kg, which are then forged, rolled on a slug and austenitized at 1130°C, water. The temperature range of hot deformation is 950-1180°C. Samples are made from heat-treated rods, which are kept at a temperature of 580 ° C for 2000 hours, then subjected to corrosion, mechanical tests and metallographic studies according to the following methods:

- the rate of pitting corrosion is determined in a solution of 10% FeCl ₃ ⋅6H ₂ O in accordance with GOST 9.912-89;

- tests for resistance to MCC are carried out according to the AMU method, GOST 6032-2003, using provocative heating of samples at a temperature of 650 ° C for 1 hour;

- static tensile tests at temperatures of 580 and 20°C are carried out, respectively, according to GOST 9651-84 and GOST 1497-84;

- the presence of secondary phases in the steel structure is determined by the metallographic method using optical and electron microscopes, using structural diagrams of Scheffler, Potak-Sagalevich and Fe-Cr-Ni ternary systems.

From tables 1 and 2 it can be seen that the value of ECTS of all heats of the claimed steel is at a level above 90, which characterizes its satisfactory thermal stability during heating at 580°C for 2000 hours and the absence of sensitization of the structure associated with the release of secondary phases. This ensures the resistance of steel against local types of corrosion, in contrast to the prototype, which is confirmed by the results of corrosion tests presented in table 2. The values of ESPS of all investigated melts do not exceed 1.10 (table 1), which indicates the preservation of resistance to sigmatization of the claimed steel and is confirmed by the absence of the sigma phase in it when considered in the triple structural diagram of the Fe-Cr-Ni system.

According to the Scheffler and Potak-Sagalevich diagrams, delta-ferrite is also absent in the melting structure of the inventive steel.

The strength properties of the proposed steel (tensile strength σv and nominal yield strength σ _0.2 ) at temperatures of 20 and 580°C, as follows from table 2, is higher than that of the prototype, and ductility (relative elongation δ ₅ ) remains at the level of the prototype.

Claims (5)

Unstabilized corrosion-resistant austenitic steel containing carbon silicon, manganese, chromium, nickel, molybdenum, nitrogen, vanadium, tungsten, boron, iron and inevitable impurities, characterized in that it additionally contains calcium and cerium in the following ratio, wt.%: carbon no more 0.04 silicon 1.2-1.9 manganese from more than 3.0 to 5.0 chromium from more than 25.0 to 28.0 nickel 20.0-23.0 molybdenum 2.0-2.5 nitrogen 0.10-0.21 vanadium 0.02-0.12 tungsten 0.10-0.20 boron 0.0005-0.008 calcium 0.001-0.015 cerium 0.001-0.005 iron and inevitable impurities rest, when the ratios are fulfilled: (1.5[Cr]+1.2[Mn]+1.1[Mo]+2.4[V]+0.5[W]-1.7[Si]-1.2[Ni]- 2.3[B])/([N]+[C])≥90; ([Cr]+[Mo]+1.5[Si]+0.5[W]+[V])/([Ni]+0.5[Mn]+30[N]+30[C]) ≤1.10.