RU2573161C1 - Nonmagnetic rust-proof steel and article made thereof - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed composition contains the following substances, in wt %: carbon ≤0.03, manganese 1.0-2.0, chromium 14.0-18.0, nickel 8.5-14.5, nitrogen 0.06-0.35, cerium 0.001-0.05, silicon 2.0-4.5, molybdenum 2.5-4.5, iron and unavoidable impurities making the rest. The following ratios are to be held: (Cr+0.75Si+Mo)/(2Ni+0.5Mn+40(N+C)) = 0.35-1.30.

EFFECT: high rust-proof ability, acceptable physical and mechanical properties.

8 cl, 5 tbl

Description

The invention relates to the field of metallurgy, to compositions of corrosion-resistant non-magnetic (austenitic) steels of increased strength, and to products made from it, for operation in medium and high aggressive environments, both oxidizing (strong nitric acid, etc.) and reducing (hydrochloric sulfuric acid) character. The invention can be used in the production of flat types of metal; hot-rolled (hot-smoked) and cold-rolled (cold-smoked) sheets; high-quality types of hire; hot rolled, cold rolled and hot pressed pipes; forgings of any configuration; welded structures, including tanks for work under pressure.

Known steels having high corrosion resistance against concentrated nitric acid at temperatures up to 100 ° C: 02Х8Н22С6 (ЭП 794), 015Х14Н19С6Б-ВИ (ЧС110-ВИ), 10Х15Н9С3Б (ЭП 302).

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

Sheets, rods, pipes, forgings and other types of metal products are made from steel.

Steels are characterized by high corrosion resistance upon contact with boiling nitric acid of high concentrations (oxidizing environment).

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, as well as relatively low corrosion resistance in reducing media, including in the presence of chlorine ions (Cl ^-1 ).

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

Carbon 0.01-0.06 Manganese 0.5-2.0 Silicon 0.1-0.8 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. $$\frac{\text{Cr}+\text{one}\text{, 2Si}}{\text{2Mn}+\text{2}\text{,5}\left(\text{Ni}-\text{0}\text{,5}\right)+\text{fifty}\left(\text{C}+\text{N}\right)}=0.40÷0.70$$

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 an increased 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 of the invention - steel and product).

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

The problem to which the invention is directed, is to create weldable, non-magnetic corrosion-resistant steel and products made from it, providing increased strength (σ _0.2 ≥325 N / mm ² ), corrosion resistance as in high oxidizing ability media (for example, in boiling nitric acid), and in reducing media containing chlorine ions.

The technical result of the invention is the creation of non-magnetic corrosion-resistant steel, providing high corrosion resistance against general and intergranular corrosion in highly oxidizing environments (boiling nitric acid of various concentrations) and in chloride-containing reducing (hydrochloric, sulfuric, sulfuric acid) media while maintaining a complex of physical mechanical properties at the prototype level.

The specified technical result is achieved in that the non-magnetic corrosion-resistant steel containing carbon, manganese, silicon, chromium, nickel, nitrogen, cerium, characterized in that it additionally contains molybdenum in the following ratio of components, wt.%:

Carbon ≤0.03 Manganese 1.0-2.0 Chromium 14.0-18.0 Nickel 8.5-14.5 Nitrogen 0.06-0.35 Cerium 0.001-0.05 Silicon from more than 2.0 to 4.5 Molybdenum 2.5-4.5 Iron and inevitable impurities rest,

, а также тем, что изделия выполнены из немагнитной коррозионно-стойкой стали указанного состава.the relation

, as well as the fact that the products are made of non-magnetic corrosion-resistant steel of the specified composition.

Products can be made in the form of hot-rolled sheets 4–40 mm thick, cold-rolled sheets 0.5–3.9 mm thick, cold-rolled strips 0.05–2.0 mm thick, in the form of hot-rolled or cold-rolled long products, in the form of hot-rolled or cold rolled seamless or welded pipe or tube billet

The essence of the invention lies in the fact that for the implementation of the technical result, the ratio of the elements responsible for the increase is regulated: strength parameters (σ _in , σ ₀₂ ), corrosion resistance in aggressive environments of an oxidizing and reducing nature and at the same time reducing the tendency to intergranular corrosion, including . welded joints, molybdenum is introduced into the composition of the steel and the silicon content increases.

The above contents of alloying elements in the proposed steel make it possible to obtain non-magnetic, corrosion-resistant steel of increased strength, suitable for the manufacture of lightweight structures, including welded, in contact with medium and high aggressiveness mediums of both oxidative and chloride-containing reducing nature .

The carbon content in steel in an amount of less than 0.03% provides resistance to intergranular corrosion, including welded joints, and improves the weldability of steel.

The accepted limits of alloying with chromium of 14.0-18.0% provide the ability of steel to passivate in a wide range of potentials characteristic of reducing and oxidizing media. A decrease in chromium content below 14% negatively affects the resistance of steel to pitting corrosion. An increase in chromium content above 18% enhances the tendency of steel to form 5-ferrite in the structure of the high-temperature phase, which has a negative effect on the workability of steel during hot processing and on the increase in magnetic permeability on the other hand.

The limits for the nickel content of 8.5-14.5% are selected based on the requirements to ensure a stable austenitic structure when alloyed with ferrite-forming elements: chromium, silicon and molybdenum. Alloying with nickel in an amount of less than 8.5% leads to a decrease in the stability of austenite during cold deformation and a decrease in temperature. An increase in the nickel content in nitrogen-containing steel above 14.5% is not economically feasible.

превращению, повышает стойкость стали практически против всех видов коррозии (межкристаллитной (МКК), точечной, ножевой и др.). Указанные свойства проявляются при легировании азотом в количестве с 0,06%. Ограничение верхнего предела по содержанию азота 0,35% вызвано пределом его растворимости при атмосферном давлении в системе легирования Fe-Cr-Ni, во избежание появления несплошности (роста) слитка при кристаллизации.To implement these qualities, nitrogen is also introduced into steel in amounts of 0.06-0.35%. Moreover, in this composition, the effect of nitrogen appears in three directions. Nitrogen, which belongs to the interstitial elements and is the strongest austenite-forming element, being in a solid γ-solution, causes matrix hardening, contributes to an increase in the stability of austenite with respect to

transformation, increases the resistance of steel against almost all types of corrosion (intergranular (MKK), pitting, knife, etc.). These properties are manifested when doping with nitrogen in an amount from 0.06%. The limitation of the upper limit on the nitrogen content of 0.35% is caused by the limit of its solubility at atmospheric pressure in the Fe-Cr-Ni alloying system, in order to avoid the appearance of discontinuity (growth) of the ingot during crystallization.

By increasing the silicon content in steel, an increased level of corrosion resistance of steel in strongly oxidizing media (boiling nitric acid, etc.) is achieved, and, most importantly, in the presence of 6-valent chromium ions (Cr ⁶⁺ ). A significant increase in corrosion resistance in oxidizing environments occurs when more than 2% silicon is introduced into steel. But an increase in silicon content of more than 4.5% in steel is undesirable, since the conditions for the formation of boundary precipitates of silicides in the metal structure of the welded joint are facilitated, which negatively affects the plastic properties of the metal.

The introduction of molybdenum into steel in amounts of 2.5-4.5%) is dictated by the fact that, in contrast to chromium, it passivates the steel surface both in aggressive environments of a reducing nature (hydrochloric, sulfuric, sulfurous, and other acids), and in highly oxidizing hydrochloric environments, and especially when chlorine ions are present in them. The limitations of the alloying limits for steel with molybdenum are explained by the fact that when alloying in an amount of less than 2.5%, its passivating ability is not enough manifested, and the introduction of more than 4.5% molybdenum into steel can be in the steel of this composition at temperatures of about 600-750 ° C , lead to the formation of intermetallic phases of the Fe ₂ Mo type (Laves phase), which reduce the enriching effect of molybdenum in steel and reduce the plastic properties.

Microalloying with cerium increases the strength and ductility of steel, while the minimum effect is achieved with the introduction of 0.001% cerium. The introduction of cerium in an amount greater than 0.05% is impractical due to the appearance of a significant amount of cerium oxides.

The content in the composition of the proposed manganese steel up to 2% provides the possibility of steel smelting by traditional methods on conventional charge materials and at the same time allows to obtain the required level of hard-soluble hardening of the austenitic base in combination with sufficient corrosion resistance.

Due to the complex influence and interaction of the main alloying elements in the Fe-Cr-Ni-Mo-Si-N system, to ensure the above technical result, it is necessary to observe the ratio of ferrite-forming and austenite-forming elements according to the formula

With a value of the specified ratio within the range of 0.35–1.30 in steel, a combination of stability of the austenitic structure, high strength and ductility at normal temperature, the necessary processability at hot deformation temperatures and the corrosion resistance of welded joints in contact with an oxidizing aggressive medium are achieved.

Examples of carrying out the invention.

The experimental steels within the claimed composition, as well as the prototype, were smelted in a vacuum induction furnace with metal spill into molds for ingots weighing 30 kg. The chemical composition of the steels is given in table 1.

The ingots were forged into billets and casing, which were rolled into bars with a diameter of 14 mm and a hot-rolled sheet with a thickness of 4-6 mm. Forging ingot heating 1150-1180 ° C. Forging of ingots onto a billet of intermediate size (a square with a side of 35 mm) and a slider was carried out in the temperature range 1150–900 ° C.

The blanks are a square with a side of 35 mm, rolled onto a bar with a diameter of 14 mm and quenched from 1060 ° C in water. Samples were made from hardened hot rolled bars to determine mechanical properties and corrosion tests. Products in the form of cold-rolled sheets with a thickness of 1.5-2.0 mm were made by cold rolling of hot-rolled sheets, which were previously quenched at 1060 ° C and etched with an alkaline-acid method.

Tables 2–5 show the results of testing the mechanical and corrosion properties of new steel.

The data in table 2 indicate that the properties of strength (σ _in and σ ₀₂ ) and ductility (δ ₅ and ψ) of the new steel in the quenched state are at the level of the same properties of the prototype. The sensitivity to stress concentration of the new steel is significantly lower than that of the prototype, and this is a positive quality.

Table 2a shows the properties of the new steel after provoking holidays at temperatures of 450-650 ° C for 1 hour, i.e. in a state of sensitization.

.The data in table 2a show that the mechanical properties of the new steel after provoking tempering practically do not differ from the properties in the quenched state, see table 2. After these tempering, there is no decrease in strength and plastic properties in steel, which indirectly indicates that the new steel after welding does not require heat treatment. Also, in the new steel, there is no increase in sensitivity to stress concentration in a sensitized state. This is evidenced by the data on plasticity (δ ₅ , ψ) and the ratio $$\frac{{\sigma }_{\mathrm{at}}{}^n}{{\sigma }_{\mathrm{at}}}$$

.

The test results of the new steel for corrosion resistance are given in tables 3, 4, 5.

Tests for resistance to intergranular corrosion (MCC) were carried out in a solution of 27% HNO ₃ +40 g / l Cr ⁶⁺ , which is an analogue of the solution for the DU method GOST 6032, but has a stronger oxidizing ability. The test results showed that all the studied steels, including the prototype, are not prone to MCC in both hardened and sensitized, i.e. after provocative holidays in the temperature range 450-650 ° C, conditions.

The presented results of mechanical and corrosion tests indicate that the technical results in the new steel are fully realized:

- steel retains an increased complex of strength and ductility at the prototype level;

- significantly increases the corrosion resistance in environments of oxidizing and reducing nature in comparison with the prototype;

- steel retains a complex of mechanical and corrosion properties in a sensitized state.

Thus, the proposed corrosion-resistant steel has a set of service properties, which allows it to produce parts and structural products from it to work in contact with aggressive environments of a reducing and strongly oxidizing nature, i.e. steel demonstrates versatility with respect to applications.

Claims (8)

1. Non-magnetic corrosion-resistant steel containing carbon, manganese, silicon, chromium, nickel, nitrogen, cerium, iron and inevitable impurities, characterized in that it additionally contains molybdenum in the following ratio of components, wt.%:
carbon ≤0.03 manganese 1.0-2.0 chromium 14.0-18.0 nickel 8.5-14.5 nitrogen 0.06-0.35 cerium 0.001-0.05 silicon from over 2.0 to 4.5 molybdenum 2.5-4.5 iron and inevitable impurities rest,
when the relation

. 2. A product made of non-magnetic corrosion-resistant steel, characterized in that it is made of steel according to claim 1. 3. The product according to p. 2, characterized in that it is made in the form of a hot-rolled sheet with a thickness of 4-40 mm 4. The product according to p. 2, characterized in that it is made in the form of a cold-rolled sheet with a thickness of 0.5-3.9 mm 5. The product according to p. 2, characterized in that it is made in the form of a cold-rolled tape with a thickness of 0.05-2.0 mm 6. The product according to p. 2, characterized in that it is made in the form of hot-rolled or cold-rolled long products. 7. The product according to claim 2, characterized in that it is made in the form of hot-rolled or cold-rolled seamless or welded pipes. 8. The product according to p. 2, characterized in that it is made in the form of hot-rolled or cold-rolled seamless or welded pipe billets.