RU2499075C1 - Corrosion-resistant austenitic steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to corrosion-resistant austenitic steels with increased content of silicon to be used in nuclear power engineering at manufacture of heat-exchange equipment operating at high temperature in contact with steam-water medium and heavy lead liquid-metal heat carrier, and namely for production of heat-exchange thin-wall tubes operating at the temperature of 550°C. Steel contains the following, wt %: carbon 0.005 - 0.04, silicon 1.6 - 3.0, manganese 0.5 - 2.0, chrome 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 are the rest.

EFFECT: improving resistance to corrosion cracking in chlorine-containing medium, as well as improving impact strength, ductility and strength at maintaining the level of corrosion resistance in heavy liquid metal heat carrier.

1 tbl, 3 ex

Description

The invention relates to metallurgy, and in particular to corrosion-resistant austenitic steels with a high content of silicon. These steels are used in nuclear energy in the manufacture of heat-exchange equipment operating at high temperature in contact with a steam-water medium and a heavy lead liquid metal coolant, for example, for the manufacture of heat-exchange thin-walled pipes operating at 550 ° C.

Known corrosion-resistant austenitic 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 unavoidable impurities rest

when performing the following ratios:

$$\frac{\%V+\%Ce+\%Ca}{\%C+\%N}=0.25-0.65$$

$$0.5<\frac{\%Ni+0.5\%Mn}{\%Cr+\%Mo+\mathrm{1,5}\%Si+\%V}<0.90$$

% Ni + 16 (% C +% N) - (% Cr + 1,5% Mo- 20) ^2/12 = 14-24,

(RF patent No. 2409697, IPC C22C 38/58, C22C 38/46, publ. 01.20.2011).

A disadvantage of the known steel is its low resistance to liquid metal lead coolant, which is explained by the low silicon content that does not create a protective oxide film on the steel surface, and also due to the increased concentration of nickel, which dissolves in liquid lead.

Known corrosion-resistant austenitic steel containing wt.%:

carbon ≤0.03 manganese 0.07-2.0 kemney 2.2-4.5 chromium 14.0-16.0 nickel 8.5-11.5 nitrogen 0.1-0.25 zirconium 0.05-0.12

iron and unavoidable impurities - the rest,

while the content of chromium, silicon, nickel and nitrogen is related

, $$\frac{Cr+0.75Si}{2.2Ni+40N}=0.55-0.80$$

,

and the content of carbon, nitrogen and zirconium is related

$$\frac{\mathrm{thirty}C+\mathrm{fifty}N}{\mathrm{eighteen}Zr}=\mathrm{four}-9$$

(RF patent No. 2432413, IPC C22C 38/58, C22C 38/50, publ. 10/27/2011).

A disadvantage of the known steel is its low resistance to corrosion cracking in aqueous environments containing chlorides.

The closest set of essential features to the invention is corrosion-resistant austenitic steel containing carbon, silicon, manganese, chromium, nickel, niobium (Technical specifications TU 14-1-1174-75 "Billet for seamless pipes from steel grade IX15H96361-I1I ( EP302-Sh) ”1975, p. 1, table 1).

Known steel EP302-Sh contains the above ingredients in the following ratio, wt.%:

carbon 0.08 ÷ 0.12 silicon 2.2-3.0 manganese 0.4-0.8 chromium 14.0-16.0 nickel 8.0-10.0 niobium 0.7-1.0 sulfur no more 0,025 phosphorus no more 0,035 iron rest.

A disadvantage of the known corrosion-resistant austenitic steel is its low resistance to corrosion cracking in chloride-containing environments, as well as low impact strength, ductility and strength.

The problem solved by the present invention is to create a corrosion-resistant austenitic steel for elements of nuclear installations with heavy liquid metal coolant, operating at temperatures up to 550 ° C in contact with molten lead coolant and steam-water medium.

The technical result of the present invention is to increase the resistance to corrosion cracking in a chloride-containing medium, as well as to increase the impact strength, ductility (elongation, 85) and strength while maintaining the level of corrosion resistance in a heavy liquid metal coolant.

The specified technical result is achieved by the fact that the known corrosion-resistant austenitic steel containing carbon, silicon, manganese, chromium, nickel, niobium,

according to the invention additionally contains molybdenum, nitrogen, vanadium, tungsten and boron in the following ratio of ingredients, mass. %:

carbon 0.005-0.04 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 unavoidable impurities - the rest

and the relations are satisfied:

, $$\frac{[Cr]+[Mo]+\mathrm{1,5}[Si]+0.5[Nb+W]}{[Ni]+\mathrm{thirty}[C+N]+0.5[Mn]}=0.95-1.25$$

,

where [Cr] + [Mo] +1.5 [Si] +0.5 [Nb + W] ≤28.5 and

[Ni] +30 [C + N] +0.5 [Mn] ≤27.0

and when performing the following dependency:

[Cr] +3.3 [Mo + Si] +16 [N] + 0.5W≥35

The introduction of molybdenum provides sufficient resistance of steel against corrosion cracking in chloride-containing environments. The value of the upper limit of molybdenum, equal to 3.8%, is associated with the need to prevent the appearance of the sigma phase, as well as with the formation of high-temperature heating in the austenitic structure of delta ferrite steel, which has a negative effect on its processability during hot deformation. When the molybdenum content is less than 1.8%, the steel becomes prone to corrosion cracking in chloride-containing environments, and its weldability is also deteriorated.

The introduction of nitrogen allows you to influence the stability of austenite, the resistance of steel to corrosion cracking and its strength. The maximum nitrogen content of 0.4% is determined by its solubility limit during the crystallization of steel at normal atmospheric pressure. When the nitrogen content is less than 0.1%, the required level of resistance to corrosion cracking and strength is not achieved.

The introduction of vanadium promotes the formation of finely dispersed nitrides, which perform the function of additional (to solid solution) hardening. The upper limit on the content of vanadium is limited to 0.5% so that the maximum amount of nitrogen can remain in the solid solution. When the vanadium content is less than 0.01%, the amount of the formed dispersed phase in the steel of the proposed composition is not sufficient for its hardening.

The introduction of tungsten regulates the content of the ferrite phase in the steel and contributes to an increase in its strength at 550 ° C. Tungsten in the range from 0.1% to 1.0% regulates the content of the ferritic phase in steel. When the tungsten content is more than 1.0%, the amount of the ferrite phase increases excessively, which leads to a decrease in the mechanical properties of steel, and when the tungsten content is less than 0.1%, its effect is ineffective.

Niobium within the declared limits is introduced in order to form finely dispersed nitrides that limit the growth of austenite grain during heat treatment. The introduction of niobium in excess of 0.5% leads to the binding of nitrogen to large nitrides, a decrease in the concentration of nitrogen in the solid solution, and an increase in the tendency of steel to corrosion crack.

The introduction of boron into steel helps to clean grain boundaries, increase technological plasticity and impact strength. When the boron content is more than 0.008%, excess phases are formed in the steel - borides, which reduce the technological plasticity of the steel, and the boron content of less than 0.0005% does not give a positive result.

The upper limit on carbon content is limited to 0.04% to eliminate the tendency to intergranular corrosion of steel and its embrittlement after cold deformation and heat aging. When the carbon content is less than 0.005%, the profitability of steel production is reduced due to a significant increase in the cost of the charge and the complexity of the technology of metal smelting.

An increase in the lower limit of silicon content to 1.6% provides satisfactory weldability of steel and its corrosion resistance in the flow of a heavy liquid metal coolant due to the formation of a stable protective oxide film on the metal surface. Alloying with silicon of more than 3.0% causes embrittlement of steel due to the formation of silicates released at the austenite grain boundaries.

The manganese content in the range from 0.5% to 2.0% is determined by its amount necessary for high-quality deoxidation of steel, sufficient assimilation of nitrogen and satisfactory weldability of steel. At a higher concentration of manganese, the resistance of steel against local corrosion in chloride-containing environments decreases.

The maximum chromium content of 21.0% is associated with the need to prevent the appearance of the sigma phase, as well as the formation of delta ferrite steel during high temperature heating in the austenitic structure, which has a negative effect on its processability during hot deformation. When the chromium content is less than 18.2%, the steel becomes prone to corrosion cracking in chloride-containing environments.

The nickel content in the range from 13.0% to 18.0% is due to the need to ensure a stable austenitic steel structure, high resistance to cracking in a steam-water medium, and high impact strength. At a nickel concentration of more than 18.0%, the corrosion resistance of steel in liquid lead decreases.

The content limits of alloying elements are determined based on the test results of the proposed steel of various chemical composition variants, as well as on the basis of structural diagrams, taking into account the role of individual components in the structural formation of steel. Ratio

, $$\frac{[Cr]+[Mo]+\mathrm{1,5}[Si]+0.5[Nb+W]}{[Ni]+\mathrm{thirty}[C+N]+0.5[Mn]}=0.95-1.25$$

,

within the stated limits, it provides steel with a stable austenite structure in heat-treated and cold-deformed states. If this ratio is over 1.25, the structure becomes unstable, which leads to the formation of a martensitic phase during cold deformation of steel, which reduces its resistance to corrosion cracking. When the value of the specified ratio below 0.95, the required set of mechanical properties and corrosion resistance of steel is not provided.

The ratio

[Cr] + [Mo] +1.5 [Si] +0.5 [Nb + W] ≤28.5 and

[Ni] +30 [C + N] +0.5 [Mn] ≤27.0

allow to obtain structurally stable austenite, economically doped with its constituent ingredients.

Dependence

[Cr] +3.3 [Mo + Si] +16 [N] +0.5 W≥35

provides steel that is not prone to corrosion cracking in a chloride-containing medium and with high corrosion resistance in a heavy liquid metal coolant.

The invention is illustrated in the table, which presents the test results of the inventive steel and prototype.

Examples of carrying out the invention.

Example 1

The claimed steel is melted with the following content of ingredients, mass%: carbon - 0.04; silicon - 2.2; manganese - 1.4; chrome 19.2; nickel - 17.7; molybdenum - 2.37; niobium - 0.50; vanadium - 0.25; tungsten - 0.42; boron - 0,0006; nitrogen - 0.19

and relations

; $$\frac{[Cr]+[No]+\mathrm{1,5}[Si]+0.5[Mb+W]}{[Ni]+\mathrm{thirty}[C+N]+0.5[Mn]}=\mathrm{one}$$

;

[Ni] +30 [C + N] +0.5 [Mn] = 25.3;

[Cr] +3.3 [Mo + Si] +16 [N] + 0.5W = 37.5

Example 2

Steel is melted with the following ingredients, mass%: carbon - 0.02; silicon - 2.8; Manganese - 0.8; chrome 18.4; nickel - 13.1; molybdenum - 3.80; niobium - 0.20; vanadium - 0.40; tungsten - 0.12; boron - 0.0020; nitrogen - 0.25 and ratios

; $$\frac{[Cr]+[No]+\mathrm{1,5}[Si]+0.5[Mb+W]}{[Ni]+\mathrm{thirty}[C+N]+0.5[Mn]}=1.23$$

;

[Cr] + [Mo] +1.5 [Si] +0.5 [Nb + W] = 26.56;

[Ni] +30 [C + N] +0.5 [Mn] ≤21.6;

[Cr] +3.3 [Mo + Si] +16 [N] + 0.5W≥44.2.

Example 3

Steel is melted with the following ingredients, mass%: carbon - 0.01; silicon - 1.9; Manganese - 1.8; chrome 21.0; nickel - 15.0; molybdenum - 1.90; niobium - 0.08; vanadium - 0.05; tungsten - 0.82; boron - 0.0040; nitrogen - 0.36 and ratios

; $$\frac{[Cr]+[No]+\mathrm{1,5}[Si]+0.5[Mb+W]}{[Ni]+\mathrm{thirty}[C+N]+0.5[Mn]}=0.97$$

;

[Cr] + [Mo] +1.5 [Si] +0.5 [Nb + W] = 26.2;

[Ni] +30 [C + N] +0.5 [Mn] = 27;

[Cr] +3.3 [Mo + Si] +16 [N] + 0.5W = 39.7

To conduct a comparative analysis for all three examples, the prototype steel is melted with the contents of the ingredients, mass%: carbon - 0.10; silicon - 2.2; Manganese - 0.7; chrome - 15.0; nickel - 9.5; niobium - 0.90.

Corrosion resistant austentin steel is prepared as follows.

Steel is smelted in an open vacuum induction furnace and poured into ingots of 17 kg, which are then forged, rolled on a stoop and subjected to austenization according to the regime of 1130 ° C, water. The temperature range of hot plastic deformation is 950-1180 ° C.

As can be seen from the table, the mechanical tests of the manufactured samples at room temperature and at 550 ° C, as well as tests of the manufactured Oding rings for resistance to corrosion cracking at room temperature in a 10% solution of FeCl · 6H ₂ O at a voltage of 1.2σ _{0 , 2} , showed that the inventive steel significantly exceeds the prototype in terms of resistance to corrosion cracking in a chloride-containing medium - 10% FeCl ₃ · 6H ₂ O at a stress of 1.2σ _0.2 , impact strength (KCV), ductility (δ ₅ ), temporary tear resistance a, and the conditional yield strength σ _0.2 at room and elevated temperatures. The corrosion resistance of steel in a heavy liquid metal coolant, which is directly dependent on the content of silicon and other elements indicated by the dependence [Cr] +3.3 [Mo + Si] +16 [N] + 0.5W≥35, remains at the prototype level.

Steel Conventional No. of swimming trunks Time to steel failure in 10% FeCl ₃ · 6H ₂ O with a voltage of 1.2σ _0.2 , h Mechanical properties Test temperature, ° С twenty 550 σw, N / mm ² σ _0.2 N / mm ² δ ₅ ,% KCV, J / cm ² σ in N / mm ² σ _0.2 > N / mm ² % Proposed one 2000 * 760 460 60 307 582 251 48 2 2000 * 737 435 72 314 545 220 51 3 2000 * 744 430 70 340 530 215 42 Prototype four 70 730 350 40 140 460 208 35 Note: * - the samples were not destroyed.

Claims (1)

Corrosion-resistant austenitic steel containing carbon, silicon, manganese, chromium, nickel, niobium, iron and inevitable impurities, characterized in that it additionally contains molybdenum, nitrogen, vanadium, tungsten and boron in the following ratio of ingredients, wt.%:
carbon 0.005-0.04 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 fulfilling the ratio:
$$\frac{[Cr]+[Mo]+\mathrm{1,5}[Si]+0.5[Nb+W]}{[Ni]+\mathrm{thirty}[C+N]+0.5[Mn]}=0.95-\mathrm{1.25,}$$

where [Cr] + [Mo] +1.5 [Si] +0.5 [Nb + W] ≤28.5 and
[Ni] +30 [C + N] +0.5 [Mn] ≤27.0
and when performing the following dependency:
[Cr] +3.3 [Mo + Si] +16 [N] + 0.5W≥35.