RU2432413C1 - Austenite corrosion-resistant steel and item manufactured of it - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains carbon, silicon, manganese, chromium, nickel, nitrogen, zirconium, iron and unavoidable impurities at following ratio of components, wt %: carbon ≤ 0.03, manganese 0.07 - 2.0, silicon 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. Contents of chromium, silicon, nickel and nitrogen are determined by dependence (Cr+0.75Si)/(2.2Ni+40N)=0.55÷0.8, while contents of carbon, nitrogen and zirconium are determined with dependence (30C+50N)/18Zr=4÷9.

EFFECT: raised resistance to general and inter-crystal corrosion in boiling nitric acid of various concentration; fabrication of qualitative welded connections at maintained level of strength and processability; increased efficiency of composition.

5 cl, 3 tbl

Description

The invention relates to the field of metallurgy, to compositions of corrosion-resistant austenitic steels of increased strength and to products made from it, and can be used in the manufacture of sheet and pipe parts, welded structures, for example, in the form of containers in contact with boiling nitric acid.

Known steels with high corrosion resistance against concentrated nitric acid at temperatures up to 100 ° C: 02Х8Н22С6 (ЭП794) and 015Х14Н19С6Б-ВИ (ЧС110-ВИ). Chemical composition of steels:

02X8H22C6 (EP 794) 015X14N19S6B-VI (ChS 110-VI) Carbon ≤0.02 Carbon≤0.015 Manganese≤0.06 Silicon 5.0-6.0 Silicon 5.4-6.7 Chrome 13.0-15.0 Chrome 7.5-10.0 Nickel 18.0-20.0 Nickel 21.0-23.0 Niobium 0.10-0.25 Sulfur≤0.02 Sulfur ≤0.020 Phosphorus≤0.03 Phosphorus ≤0.030 Iron is the basis Titanium≤0.05 Iron is the basis

Sheets, rods, pipes are made of steel.

(Reference book “Corrosion-resistant, heat-resistant and high-strength steels and alloys”, pp. 133-137, M., 2008)

Steel is characterized by high corrosion resistance in contact with boiling nitric acid of high concentrations.

The disadvantage of these steels is reduced strength, especially in terms of yield strength σ _0.2 (175-245 N / mm ² ), which prevents their use in highly loaded structures.

Known austenitic corrosion-resistant steel and a product made of it. Austenitic steel contains the following components, wt.%:

Carbon 0.01-0.06 Silicon 0.1-0.8 Manganese 0.5-2.0 Chromium 16.0-19.0 Nickel 8.0-10.5 Nitrogen 0.05-0.25 Boron 0.001-0.005 Calcium 0.01-0.10 Cerium 0.001-0.05 Sulfur 0,030 Phosphorus 0,045 Iron and inevitable impurities rest

when performing the following ratios:

и ∑B+Ca+Ce≈0,12.

and ∑B + Ca + Ce≈0.12.

The product can be made in the form of hot-rolled sheets with a thickness of 3.0-8.0 mm, or in the form of cold-rolled sheets with a thickness of 0.5-3.0 mm, or in the form of rods with a diameter of 4-8 mm. The steel according to this invention has a high level of strength (σ _in 705-720 N / mm ² , σ _0.2 365-395 N / mm ² ), good cold forming and resistance to general and intergranular corrosion, satisfactory weldability.

(Patent RU 2173729, publ. 09/20/2001, IPC C22C 38/54, C22C 38/58 - prototype inventions - steel and product.)

The disadvantage of the prototype is that steel and products made from them, having a high level of strength characteristics, do not provide the necessary corrosion resistance in highly oxidizing environments, in particular in boiling nitric acid.

The problem to which the invention is directed, is to create weldable corrosion-resistant steel and products made from it, providing high corrosion resistance in boiling nitric acid of sufficiently high concentrations with a relatively low content of expensive elements in combination with increased strength. In addition, steel should have good manufacturability in the manufacture of welded structures by methods of hot and cold pressure processing and low sensitivity to stress concentrators.

The technical result of the invention is to increase the resistance against general and intergranular corrosion in boiling nitric acid of various concentrations, the possibility of obtaining high-quality welded joints while maintaining the level of strength and manufacturability. An additional result is also an increase in the economy of the composition.

The specified technical result is achieved in that the austenitic corrosion-resistant steel for working in contact with boiling nitric acid, containing carbon, chromium, silicon, manganese, nickel, nitrogen, according to the invention additionally contains zirconium in the following ratio, wt.%:

Carbon ≤ 0.03 Manganese 0.07-2.0 Silicon 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 inevitable impurities rest

, а содержание углерода, азота и циркония связано зависимостью

, а также тем, что изделия, работающие в контакте с азотной кислотой, выполнены из аустенитной коррозионно-стойкой стали указанного состава. Изделия могут быть выполнены в виде листов горячекатаных толщиной 4-20 мм, холоднокатаных листов или прутков толщиной 0,8-3 мм и трубной заготовки.the content of chromium, silicon, nickel, nitrogen is related by

, and the content of carbon, nitrogen and zirconium is related

, as well as the fact that the products working in contact with nitric acid are made of austenitic corrosion-resistant steel of the specified composition. Products can be made in the form of hot-rolled sheets with a thickness of 4-20 mm, cold-rolled sheets or rods with a thickness of 0.8-3 mm and a tube billet.

The essence of the invention lies in the fact that the ratio of the elements responsible for increasing corrosion resistance and strength characteristics and at the same time reducing sensitivity to stress concentrators is regulated in steel; increased silicon content and introduced zirconium.

The implementation of these qualities of steel occurs when the amount of introduced nitrogen after crystallization of the ingot is 0.1-0.25%. Moreover, in this composition, the effect of nitrogen appears in three directions. Nitrogen, being in solid γ-solution, causes matrix hardening. At the same time, as a result of interaction with silicon, the solubility of nitrogen in austenite decreases to some extent, which contributes to the initial stage of pre-precipitation of the carbonitride phase, which creates an additional reserve of hardening, and, finally, the introduction of nitrogen helps to stabilize the γ-solid solution. These changes begin with doping with nitrogen in an amount of 0.1%. The limitation of the upper limit on the content of 0.25% is caused by the solubility limit of it in liquid metal in order to avoid the appearance of discontinuity of the ingot.

The limits for the chromium content are selected taking into account the fact that additional doping with silicon in an amount of 2.2-4.5% compensates for the decrease in the concentration of chromium compared to the prototype in terms of increasing resistance to aggressive environments.

A decrease in the concentration of chromium has a positive effect on a decrease in the sensitivity of the metal to stress concentrators. The best results from a decrease in chromium concentration in terms of reducing sensitivity to stress concentration and at the same time maintaining corrosion resistance to general and intergranular corrosion are achieved with a chromium content of 14.0-16.0%.

By increasing the silicon content, an increased level of corrosion resistance of steel in boiling nitric acid against general and intergranular corrosion and, which is especially important, in the presence of 6-valent chromium ions (Cr ⁶⁺ ) is achieved. A significant increase in the corrosion resistance of boiling nitric acid occurs with the introduction of 2.2% silicon, but an increase in the silicon content of more than 4.5% is undesirable due to the appearance of silicides in the metal structure of the welded joint and the deterioration of plastic properties.

The limits for the nickel content are selected based on the requirements of ensuring a stable austenitic structure when doped with silicon and range from 8.5 to 11.5%. The nickel content of 8.5 is the minimum amount that provides a predominantly austenitic structure in which, after high temperature heating, the formation of delta ferrite takes place in an amount of not more than 2-3%, which does not affect the processability of the metal during pressure processing. The upper limit of nickel content is limited to 11.5% due to the fact that a further increase in the content of this element will contribute to an increase in the sensitivity of steel to intergranular corrosion in the temperature range 450-750 ° C, since with this nickel content carbon activity will noticeably increase upon provoking tempering in the specified temperature range.

Doping with zirconium is aimed at grinding the size of austenitic grain. In addition, in this composition zirconium is part of the carbonitride phase. Therefore, during prolonged use in boiling nitric acid, the enrichment of the solution with Cr ⁶⁺ ions is much slower due to the increased resistance of the carbonitride phase with zirconium (increases the period of their pre-separation stage). A decrease in zirconium content below the indicated limit of 0.05% will not be enough to grind austenite grain. An increase above 0.12% is associated with the fact that chromium-containing compounds will accumulate at grain boundaries, which adversely affects the corrosion properties of steel.

Limiting the carbon content to 0.03% provides resistance to intergranular corrosion.

Due to the complex influence and interaction of the main alloying elements in the Fe-Cr-Ni-Si-N system, to ensure the above technical result, it is necessary to introduce a compositional restriction according to the ratio:

, а также

.

, as well as

.

When the value of the first ratio is more than 0.8, manufacturability worsens and sensitivity to stress concentrators increases, and when the value is below 0.55, the corrosion resistance of welded joints decreases under operating conditions in boiling nitric acid. It was experimentally established that the limits of the magnitude of the second ratio provide the optimal combination of mechanical and corrosion parameters.

Examples of carrying out the invention

The experimental steels within the claimed composition, as well as the prototype, were smelted in the vacuum induction furnace of the TsNIIchermet experimental complex with metal casting into molds for ingots weighing 10 kg. The chemical composition is shown in table 1. The ingots were forged into strips 5 mm thick. Forging ingots were heated at 1150 ° C. Forging was carried out in the temperature range 1150-900 ° C. Strips of 5 mm thickness after quenching from 1000 ° C and alkaline acid etching were rolled in a cold rolling mill to a thickness of 2 mm, quenched from 1100 ° C in water and alkaline acid etching. Samples were made from quenched hot-rolled and cold-rolled strips to determine mechanical properties, including sensitivity to stress concentrators, as well as samples for corrosion tests.

Tables 2 and 3 show the test results.

Table 1 The chemical composition of the proposed steel and prototype Swimming trunks number C Mn Si Cr Ni N Zr S P

ratio

one 0.02 0.9 2,4 14.7 9.1 0.14 0.09 0.005 0.008 0.64 4.69 2 0.018 1,6 3.8 15.6 11.2 0.18 0.06 0.005 0.008 0.58 8.8 3 prototype 0,027 0.9 0.7 18.5 8.3 0.15 - 0.018 0.01 0.78 - Note: 1. In all 3 swimming trunks, iron and inevitable impurities - the rest
2. The prototype also contains 0.003% boron, 0.05% calcium and 0.02% cerium

table 2 The mechanical properties of the proposed steel and prototype Swimming trunks number Tensile strength σ _in , N / mm ² Tensile Strength σ _0.2 Elongation, δ ₅ ,% Relative compression in tension, Ψ,% Sensitivity characteristics to stress concentrators * (Kt = 6.0) σ _B ^H , Ψ ^H ,%

N / mm ² one 700 368 58 68 738 51 1.05 2 720 374 57 65 749 62 1,04 3 proto-
type of 720 365 60 76 650 47 0.90 Note: For most real designs, the notch on the specimen corresponding to the Neuber stress concentration coefficient Kt = 4.3-6.7 (the ratio of depth to the radius of the groove (a / r) allows us to estimate the sensitivity of the material to stress concentrators). In this case, the smaller the ratio of σ _in ^H / σ _in , the higher the sensitivity to stress concentrators

Thus, the above results of mechanical and corrosion tests indicate increased corrosion resistance of the proposed composition in comparison with the prototype with a decrease in sensitivity to stress concentrators and almost the same level of strength.

Claims (5)

1. Austenitic corrosion-resistant steel for working in contact with boiling nitric acid, containing carbon, silicon, manganese, chromium, nickel, nitrogen, iron and inevitable impurities, characterized in that it additionally contains zirconium in the following ratio of components, wt.% :
carbon ≤0.03 manganese 0.07-2.0 silicon 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 inevitable impurities rest
the content of chromium, silicon, nickel, nitrogen is related by

, and the content of carbon, nitrogen and zirconium is related

. 2. The product is in contact with boiling nitric acid, made of austenitic corrosion-resistant steel, characterized in that it is made of steel according to claim 1. 3. The product according to claim 2, characterized in that it is made in the form of hot-rolled sheets with a thickness of 4-20 mm 4. The product according to claim 2, characterized in that it is made in the form of cold-rolled sheets or rods with a thickness of 0.8-3.0 mm 5. The product according to claim 2, characterized in that it is made in the form of a pipe billet.