RU2625514C1 - Casting austenitic high-strength corrosion-resisting in inorganic and organic environments cryogenic steel and method of its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: obtain the steel melt, pour it into the mould to obtain the casting and cool it. The steel casting has the following composition, wt %: C 0.05-0.07, Cr 18.0-20.0, Ni 5.0-7.0, Mn 9.0-11.0, Mo 1.4-1.8, Si 0.25-0.35, Cu 3.6-4.0, N 0.14-0.22, Al 0.015-0.035, B 0.05-0.1, S ≤0.0025, P≤0.010, Sn≤0.005, Pb≤0.005, Bi≤0.005, As≤0.005, Fe is the rest. The nitrogen content is related to the contents of chromium, copper, nickel and manganese by the following relationships: at the ratio (Cr+5Cu)/(Ni+Mn)≤2.50, the nitrogen content is N = 0.14-0.18%, and at the ratio (Cr+5Cu)/(Ni+Mn)>2.50, the nitrogen content is N = 0.18-0.22%. The casting mechanical processing, its subsequent heating up to the temperature of 1150-1250°C, and the homogenization of at least 2 hours and hardening to the solid solution with the ferrite content in the structure of steel not more than 5%, is carried out.

EFFECT: required strength and corrosion properties in the inorganic and organic media in the cryogenic temperatures field.

2 cl,1 tbl,1 ex

Description

The invention relates to the metallurgy of structural steels containing iron as a basis with a given ratio of alloying and impurity elements, and is intended for various industries, including for the manufacture of cryogenic high-strength cast products used in the transportation of liquefied gases.

Known stainless austenitic steel (RU 2102522 C1, publ. 20.01.1998). Known steel contains carbon, chromium, nickel, manganese, nitrogen, silicon, vanadium, copper, molybdenum, cerium, selenium, iron in the following ratio, wt.%: Carbon 0.01-0.06, chromium 18-22, nickel 15 -18, manganese 2-10, nitrogen 0.2-0.5, silicon 0.01-0.45, vanadium 0.1-0.5, copper 0.1-1.5, molybdenum 0.1-2 5, cerium 0.005-0.25, selenium 0.05-0.25, iron - the rest, and with a manganese content of less than 5, the nitrogen content of about 0.3, with a manganese content of more than 5, the nitrogen content of 0.4-0.5 .

Known austenitic steel has an increased set of technological, mechanical properties, as well as the stability of the austenitic structure, and can be used for the manufacture of highly loaded parts of machines and apparatuses of cryogenic technology.

The disadvantages of this steel are as follows.

Steel is uneconomical, since it has a high content of expensive elements of nickel (up to 18%) and molybdenum (up to 2.5%). So the nickel content is higher than in the classic austenitic stainless steel 18-10. Currently, manganese and nitrogen are used to stabilize the austenitic structure. A number of steel compositions within the declared limits of the element contents cannot be realized. For example, in this steel with a manganese content of more than 5%, a nitrogen content of 0.4-0.5% is allowed. In fact, with copper contents above 1.0% and manganese less than 10%, the nitrogen content should be less than declared, since bubbles will form during solidification of the ingots at 0.4-0.5% nitrogen.

The prototype of the first object of the invention is a stainless austenitic cast steel (RU 2451763 C2, publ. 05.27.2012).

Known stainless austenitic cast steel with an aluminum content of from more than 0 to less than or equal to 4% and with a silicon content of from 0 to 4% and with tensile strengths of more than 550 MPa and elongation at break exceeding 30%, was obtained in the range defined by the coordinates of four points: (Cr _equiv. = 14; Ni _equiv. = 8), (Cr _equiv. = 14; Ni _equiv. = 14), (Cr _equiv. = 22; Ni _equiv. = 8) and (Cr _equiv. = 22; Ni _equiv. = 16), where the chromium and nickel equivalents are calculated based on the chemical composition of the cast steel, using ratios (1) and (2) (wt.%):

The remainder of the composition consists mainly of iron and impurities contained in cast steel. This cast steel under load detects the PNP effect and contains manganese from 0 to 25%, chromium from 12 to 20%, nickel from 0 to 12%, niobium from 0 to 1.2%, tantalum from 0 to 0.2%, carbon from 0.01 to 0.15%, nitrogen from 0.005 to 0.5%, copper from 0 to 4%, cobalt from 0 to 1%, molybdenum from 0 to 4%, tungsten from 0 to 3%, titanium from 0 to 1%, vanadium from 0 to 0.15%.

The disadvantages of this steel are as follows.

The range of alloying element concentrations specified according to the invention includes a large number of alloyed steels with substantially different structure and properties, that is, it does not provide specific information about the composition of the steel proposed in the invention. Most combinations of the compositions of this invention in the Scheffler diagram are in martensitic or biphasic martensite + austenite, austenite + ferrite regions, that is, they are not austenitic, which does not correspond to the objective of the invention. Steel is also uneconomical, as it contains elements such as: nickel up to 12%, molybdenum up to 4%, tungsten up to 3%, titanium up to 1%, cobalt up to 1%, niobium up to 1.2%, tantalum up to 0.2% . The invention does not specify the relationship between nitrogen, titanium and aluminum, on which the structure of the steel, the number of excess phases and the properties of the steel depend.

The prototype of the second object of the invention is a method for producing stainless austenitic cast steel (RU 2451763 C2, publ. 27.05.2012).

The method of producing stainless austenitic cast steel having a tensile strength of more than 550 MPa and elongation at break exceeding 30%, which has a PNP effect under load.

The method includes obtaining an alloy containing the components indicated above, pouring the melt into a mold, after which the resulting cast steel is subjected to heat treatment.

The disadvantage of the production method is the difficulty of implementing the method in production due to the large number of alloying expensive elements, as well as the lack of information about the heat treatment modes, which will not allow to obtain the claimed steel structure without additional research.

In the first object of the invention, the technical result is to provide high strength and corrosion properties in inorganic and organic environments in the field of cryogenic temperatures, as well as in the economy of the proposed steel due to the small number of alloying elements, in improving the processability of steel with the following characteristics:

- strength at room temperature, σ _in ≥600 MPa;

- corrosion resistance in inorganic and organic environments;

- stable austenitic structure at low temperatures, M _H ≤-240 ° C, M _d30 ≤-40.

The technical result in the first object of the invention is achieved as follows.

Austenitic foundry high-strength corrosion-resistant inorganic and organic media cryogenic steel contains carbon, chromium, nickel, manganese, molybdenum, silicon, nitrogen, aluminum, iron and impurities, as it contains sulfur, phosphorus, tin, lead, bismuth and arsenic . It additionally contains copper and boron in the following ratio of components, wt.%:

C - 0.05-0.07

Cr - 18.0-20.0

Ni - 5.0-7.0

Mn - 9.0-11.0

Mo - 1.4-1.8,

Si - 0.25-0.35

Cu - 3.6-4.0

N - 0.14-0.22

Al - 0.015-0.035

B - 0.05-0.1

S≥0.0025

P≤0.010

Sn≤0.005

Pb≤0.005

Bi≤0.005

As≤0.005

Fe is the rest.

In this case, the ferrite content in the structure is not more than 5%, and the nitrogen content is associated with the contents of chromium, copper, nickel and manganese in the following ratios: for the ratio (Cr + 5Cu) / (Ni + Mn) ≤ 2.50 it is equal to N = 0.14 -0.18%, and with the ratio (Cr + 5Cu) / (Ni + Mn)> 2.50 it is equal to N = 0.18-0.22%.

In the second object of the invention, the technical result consists in the simplicity of the implementation of this method in production, in the possibility of obtaining the required nitrogen content during smelting at normal pressure in existing units.

The technical result of the second object of the invention is achieved as follows.

A method for producing austenitic casting high-strength corrosion-resistant steel in inorganic and organic environments of cryogenic steel involves producing a steel melt, pouring the melt into a casting mold to produce a casting, cooling the casting, machining, subsequent heating of the casting to a temperature of 1150-1250 ° C, homogenization of at least 2 hours and its hardening on a solid solution.

The advantages of the steel proposed in the invention and the method for its preparation and processing is that when the declared content of the main elements is C, Cr, Ni, Mn, Mo, Cu, B and the nitrogen content is selected in the ratio N = 0.14-0.18% with a ratio of (Cr + 5Cu) / (Ni + Mn) ≤ 2.50 and 0.18-0.22% with a ratio of (Cr + 5Cu) / (Ni + Mn)> 2.50, the equilibrium basic structure of steel in the recommended the temperature range of the heat treatment consists of a γ-phase, which is guaranteed to provide in real technological conditions pure austenitic or austenitic with a content of not more than 5% ferrite structure the desired set of properties.

The proposed steel is also highly economical, as it has a small number of non-deficient alloying elements, a low nickel content, and high processability, since the required nitrogen content can be obtained by smelting at normal pressure in existing units.

The carbon content in the range of 0.05-0.07% contributes to the formation of an austenitic structure in steel, provides, together with nitrogen, the necessary hardening of the steel during heat treatment and sufficient corrosion resistance. With a higher carbon content in steel, the corrosion resistance decreases, the tendency to MCC increases, and the tendency to brittle fracture increases.

Chromium, nickel, manganese and molybdenum within specified limits with a copper content of 3.6-4.0 wt.% And nitrogen 0.14-0.22 wt.% For all possible combinations of the contents of these elements in the composition range defined by the invention, provide in the finished steel, a stable pure austenitic structure, the required mechanical properties and suitability for the manufacture of cold-resistant high-strength parts used in cryogenic technology and in the transportation of liquefied gases.

When the content of alloying elements (Ni, Mn, Mo) below the claimed limit, it is impossible to achieve a purely austenitic structure and the desired properties, as well as the nitrogen content required by the invention. At high contents of these elements, although an austenitic structure is obtained, the resulting γ-solid solution has an increased level of strength. The increased content of Cr and Mo makes it difficult to obtain an austenitic structure and dissolution of excess phases. With a reduced chromium content, the corrosion resistance of steel decreases. The high content of manganese complicates the process of steelmaking, with a high content of nickel, steel is uneconomical.

With the declared content of Cr, Ni, Mn, Mo and Cu, a high solubility of nitrogen in the liquid phase and in austenite is ensured, as a result of which, with all possible combinations of the contents of the elements in the composition region defined by the invention and the nitrogen content of 0.14-0.22 mass% .% steel crystallizes without the formation of nitrogen bubbles in the casting. With a lower nitrogen content, the required mechanical and corrosive properties are not achieved, with a higher nitrogen content, nitrogen bubbles can form in the casting.

Copper gives steels of this composition increased corrosion resistance in various environments. When the copper content is 3.6-4.0 wt.%, The declared content of the main elements is C, Cr, Ni, Mn, Mo and the nitrogen content is selected by the ratio N = 0.14-0.18% with the ratio (Cr + 5Cu ) / (Ni + Mn) ≤ 2.50 and 0.18-0.22% for the ratio (Cr + 5Cu) / (Ni + Mn)> 2.50, the equilibrium basic structure of steel in the recommended range of heat treatment temperatures consists of γ -phase, which is guaranteed to provide in real technological conditions purely austenitic or austenitic with a content of not more than 5% ferrite structure and the required set of properties.

With a lower copper content in the steel structure, corrosion resistance in various environments is reduced. A higher copper content is undesirable, since the heterogeneity of the melt and, accordingly, of the finished steel in terms of chemical composition and properties is possible.

Boron in the specified range of 0.05-0.1 wt.% Contributes to the grinding of grain and prevents the formation of columnar crystals in the casting. With a lower boron content, its positive effect on the cast structure is not manifested. With a higher content at a solidus temperature, a large amount of borides is released, which adversely affects the strength and ductility of steel.

Aluminum in the specified range of 0.015-0.035 wt.% Provides the necessary degree of deoxidation of steel and oxygen content. With a lower aluminum content, the required degree of steel deoxidation is not ensured and the formation of chromium oxides is possible; a higher aluminum content leads to the formation of high-temperature aluminum nitrides, which negatively affect the properties of steel.

Silicon within the specified limits contributes to the effective deoxidation of steel and the removal of non-metallic inclusions, and also provides an acceptable value for the equivalent concentration of chromium Cr _e . With a higher silicon content, Cr _e and the amount of ferrite in the steel structure increase. With a lower silicon content, the process of steel deoxidation is hindered.

The presence of impurities makes it difficult to obtain a given structure and properties and reduces the effect of introducing nitrogen into steel. Therefore, as a rule, steels alloyed with nitrogen are smelted using pure steel technology. The content of harmful impurities required by the invention P≤0.010, S≤0.0025, Sn≤0.005, Pb≤0.005, As≤0.005, Bi≤0.005 in steel provides the highest level of properties for a given composition. With a higher content of impurities, their negative effect on the structure and properties of steel and the processes of structure formation is manifested. A significantly lower content of impurities is currently technologically difficult to implement.

The declared cast steel is smelted in conventional units and poured into the mold. After cooling and machining, the casting is heated to a temperature of 1150-1250 ° C, homogenized for at least 2 hours and then quenched with a solid solution. At these temperatures, the basic structure of the steel is either pure austenite or austenite with a low ferrite content, which remains after quenching of the casting. At temperatures above 1000 ° C, diffusion processes proceed rather quickly, however, the time to achieve a homogeneous equilibrium structure throughout the metal volume depends on the thickness of the casting wall. Actually, given the variety of forms of castings, in most cases this time should not be less than 2 hours.

In the heat treatment method according to the invention, the steel has a purely austenitic or austenitic structure with a content of not more than 5% ferrite structure and the required complex of mechanical and physical characteristics.

If the temperatures, duration of heating and cooling rate required for quenching are not observed, obtaining austenitic or austenitic with a content of not more than 5% ferrite steel and achieving its claimed properties according to the invention is impossible.

An example of the implementation of steel smelting and processing

Experimentally, the steel of the claimed composition was melted in a vacuum induction furnace with a capacity of 50 kg for molten metal. Pure charge materials were used: Armco iron, electrolytic nickel and copper, metallic chromium and manganese, pure molybdenum, ferroboron, nitrided ferrochrome.

After alloying and mixing the melt in order to average it, samples with a diameter of 20 mm were cast. The chemical composition of the obtained steel is presented in the table.

After cooling, separating the gate system and cleaning, the casting was heated to a temperature of 1150-1200 ° C, homogenized for 2 hours and cooled in water.

After that, samples were prepared from samples to study the structure and properties. Received after hardening single phase structure of γ and properties: σ _in = 680 MPa, σ _0,2 = 300 MPa.

Corrosion resistance of steel against general, IWC and pitting corrosion in sea water; 0.5 M H ₂ SO ₄ ; 0.5 M H ₂ SO ₄ + H ₂ S and in solutions with sulfate reducing bacteria was higher than that of traditional stainless steel XI8H10.

Claims (21)

1. Foundry austenitic high-strength corrosion-resistant inorganic and organic environments cryogenic steel containing carbon, chromium, nickel, manganese, molybdenum, silicon, nitrogen, aluminum, iron and impurities, as it contains sulfur, phosphorus, tin, lead, bismuth and arsenic, characterized in that it additionally contains copper and boron in the following ratio of components, wt.%: C 0.05-0.07 Cr 18.0-20.0 Ni 5.0-7.0 Mn 9.0-11.0 Mo 1.4-1.8 Si 0.25-0.35 Cu 3.6-4.0 N 0.14-0.22 Al 0.015-0.035 B 0.05-0.1 S ≤0.0025 P ≤0.010 Sn ≤0.005 Pb ≤0.005 Bi ≤0.005 As ≤0.005 Fe - the rest, the ferrite content in the steel structure is not more than 5%, and the nitrogen content is associated with the contents of chromium, copper, nickel and manganese in the following ratios: with a ratio of (Cr + 5Cu) / (Ni + Mn) ≤ 2.50, the nitrogen content is N = 0.14-0.18%, and with a ratio of (Cr + 5Cu) / (Ni + Mn)> 2.50 nitrogen is N = 0.18-0.22%. 2. A method of producing an austenitic foundry high-strength corrosion-resistant in inorganic and organic environments of cryogenic steel according to claim 1, which includes obtaining a steel melt, pouring the melt into a mold with casting, cooling the casting, machining, subsequent heating of the casting to a temperature of 1150-1250 ° C, homogenization for at least 2 hours and its hardening in a solid solution.