RU2657741C1 - Structural cryogenic austenite high-strength corrosion-resistant weldable steel and its treatment method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy, namely, to the production of ingots from structural cryogenic austenitic high-strength corrosion-resistant welded steel, for production of used in the liquefied gases transportation and storage high-strength welded cryogenic structures. Said steel contains, wt%: C 0.05–0.07, Cr 18.0–20.0, Ni 5.0–7.0, Mn 9.0–11.0, Mo 1.8–2.2, Si 0.25–0.35, N 0.30–0.38, Cu 0.6–1.4, B from more than 0.01 to 0.015, Al 0.015-0.035, S≤0.0025, P≤0.015, Sn≤0.005, Pb≤0.005, Bi≤0.005, As≤0.005, Fe – the rest. Produces ingots are heated to the temperature of 1,250–1,050 °C, forged into blanks with the total degree of deformation of at least 50 % and cooled down in air. Blanks are heated-up to the temperature of 1,180–1,080 °C with subsequent rolling with the total reduction rate of at least 40 % and air cooling with the rolled products production. Rolled products heated to the temperature of 1,150–1,080 °C and performing the final rolling in 2-3 passes with the total reduction rate of 30–80 % and the rolling completion temperature of 1,050–1,080 °C, after which performing the accelerated cooling down at a rate of 20–100 °C/s until the room temperature.

EFFECT: enabling the strength and viscosity required values in the cryogenic temperatures range, weldability and corrosion resistance in the acidic environment and sea water.

2 cl, 2 tbl

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 welded structures 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 is 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 high contents of expensive elements 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 proposed invention is a corrosion-resistant high-strength non-magnetic steel (RU 2392348 C2, publ. 06/20/2010). Steel has the following composition: carbon 0.02-0.06, silicon 0.10-0.60, manganese 9.5-12.5, chromium 19.0-21.0, nickel 4.5- 7.5, molybdenum -1.2-2.0, vanadium - 0.08-0.22, calcium - 0.005-0.010, sodium - 0.005-0.010, niobium - 0.05-0.15, magnesium - 0, 0005-0.001, nitrogen - 0.40-0.60, aluminum - 0.005-0.01, iron and impurities - the rest, while as impurities it contains sulfur 0.003-0.012 wt. %, phosphorus 0.004-0.025 wt. %, lead 0.0002-0.005 wt. %, bismuth 0.0002-0.005 wt. %, tin 0.0002-0.005 wt. %, arsenic 0.0002-0.005 wt. % and copper 0.05-0.2 wt. %

The disadvantages of this steel are as follows.

This steel crystallizes with the formation of γ and δ phases. With a significant number of combinations of the basic elements Cr, Ni, Mn, V, Nb, Mo of the chemical composition determined by this invention, the declared nitrogen content cannot be obtained by standard technology, since the proposed 0.40-0.60% nitrogen content in steel with these combinations of chemical composition, it exceeds its standard solubility in the metal at smelting temperatures, and the amount of nitrogen that can be introduced into the liquid metal at the smelting temperature exceeds its solubility in the γ and δ phases released during solidification, therefore The excess nitrogen will be released into the gas phase and form bubbles and porosity in the ingot. In addition, the simultaneous preparation of the active elements V, Nb, Al, Ca, Mg, Na in steel in the stated ratios is technically difficult and unrealizable in industrial production; misses in the analysis of these elements and loss of the properties of the finished metal are inevitable if they really depend on the contents these elements and their relationships.

The prototype of the second object of the proposed invention is a method of thermal deformation processing of corrosion-resistant high-strength non-magnetic steel (RU 2392348 C2, publ. 06/20/2010).

Method of thermo-deformation processing of corrosion-resistant high-strength non-magnetic steel, including heating the ingot, deformation of the ingot into the plate in the temperature range 1240-1000 ° C with a total degree of deformation of 40-94%, cooling the plate in air to control the quality of the surface and its cleaning, deformation of the resulting plate in the temperature range 1240-1000 ° С with fixing the total degree of deformation of 45-65% by 10-14% per passage into the sheet, the thickness of which is 2.5-3.5 times less than the thickness of the plate, the resulting sheet is air-cooled to 1000- 950 ° C, co troliruyut temperature on its surface is deformed and finally for 2-3 passes of 8-12% over a pass, followed by accelerated cooling at a rate 10-50 ° C / s to a temperature of 100-150 ° C on the surface of the sheet and subsequent cooling in air.

Known steel is smelted in furnaces using standard technology. To give steel a higher level of strength, more stable characteristics of mechanical properties, a reduced tendency to intergranular corrosion, increased wear resistance in ice conditions, improved weldability, low magnetic permeability and increased hot process plasticity, the steel is subjected to thermal deformation treatment in a special mode.

The disadvantages of the technology of thermal deformation processing are excessive detailing of operations, which impedes the implementation and control of the technology, in addition, with the recommended mode of heating before deformation, for some ratios of elements, the steel structure during heating ≥1200 ° С consists of several phases, including δ-ferrite. For example, at the highest contents of Cr, Mn, Nb and the lowest C and Ni, at a nitrogen content of 0.40-0.5% and at 1200-1320 ° C, the steel has the structure α + γ + (Nb, Cr) N. Therefore, at the claimed heating temperature according to the patent 1240 ° C, it is impossible to obtain a homogeneous γ-structure and austenitic finished steel before rolling.

In the first object of the proposed invention, the technical result consists in high strength and corrosion properties of steel in the field of cryogenic temperatures

- strength at room temperature, σ _in ≥800 MPa, σ _0.2 ≥600 MPa;

- viscosity in the region of cryogenic temperatures, KCU ^{(-170) ° С} ≥1.5 J / cm ² ;

- good weldability;

- profitability, as it has a low nickel content;

- stable austenitic structure in the temperature range - 175 ÷ 100 ° C;

- corrosion resistance in an acidic environment and sea water.

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

An ingot of structural cryogenic austenitic high-strength corrosion-resistant welded steel containing carbon, chromium, nickel, manganese, molybdenum, silicon, nitrogen, aluminum, iron and impurities, in which it contains sulfur, phosphorus, tin, lead, bismuth and arsenic, characterized in that it additionally contains copper and boron in the following ratio of components, mass. %:

C - 0.05-0.07,

Cr - 18.0-20.0,

Ni - 5.0-7.0,

Mn-9.0-11.0,

Mo - 1.8-2.2,

Si - 0.25-0.35,

N - 0.30-0.38,

Cu - 0.6-1.4,

B - from more than 0.01 to 0.015,

Al - 0.015-0.035,

S≤0.0025,

P≤0.015,

Sn≤0.005,

Pb≤0.005,

Bi≤0.005,

As≤0.005,

Fe is the rest.

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

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

The method of thermal deformation processing of an ingot according to claim 1, which consists in heating the ingot to a temperature of 1250-1050 ° C, forging it into a workpiece with a total degree of deformation of at least 50%, cooling the workpiece in air, heating the workpiece to a temperature of 1180-1080 ° C followed by rolling with a total degree of compression of at least 40% and cooling in air to produce rolled products, heating the rolled products to a temperature of 1150-1080 ° C and final rolling in 2-3 passes with a total degree of compression of 30-80% and the temperature of the end of rolling 1050-1080 ° C, then about uschestvlyayut accelerated cooling at a rate of 20-100 ° C / s to room temperature.

The advantages of the steel proposed in the invention and its processing method are that when the declared content of the main elements is C, Cr, Ni, Mn, Mo, Cu, B, the nitrogen content is N = 0.34-0.38%) with the contents of copper and boron Cu = 0.6-1.0% and B = 0.005-0.010%), and with a nitrogen content of N = 0.30-0.34% with copper and boron contents Cu = 1.1-1.4%) and B = 0.011-0.015%, the equilibrium structure of the steel in the temperature range from 1050 to 1300 ° C is austenite with small particles of chromium boride Cr ₂ B, which is guaranteed to provide the basic austenitic structure and the required structure under real technological conditions complex of properties.

The proposed steel is also characterized by high efficiency, as it has a small nickel content, and high processability, as it has a small amount of alloying elements and, at their stated contents, 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 and thermal deformation processing and sufficient corrosion resistance and weldability. With a higher carbon content in steel, corrosion resistance decreases, the tendency to MCC increases, the tendency to brittle fracture increases, and weldability deteriorates.

Chromium, nickel, manganese, molybdenum and copper within specified limits with a boron content of 0.005-0.015%) and nitrogen of 0.30-0.38% of the mass. with all possible combinations of the contents of these elements in the composition range defined by the invention, they provide a stable austenitic structure with a small amount of small particles of boron in the finished steel, the required mechanical properties, corrosion resistance in acidic medium and sea water and suitability for the manufacture of cold-resistant high-strength welded structures used when transporting liquefied gases.

When the content of the main alloying elements (Cr, Ni, Mn, Mo,) is lower than the claimed limit, it is impossible to achieve an austenitic structure and 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 during hot plastic deformation.

The increased content of Cr and Mo expands the α + γ region at high temperatures and makes it difficult to dissolve excess phases. The high content of manganese complicates the process of steelmaking, with a high content of nickel, steel is uneconomical.

When the declared content of Cr, Ni, Mn, Mo provide a high solubility of nitrogen in the liquid phase and in austenite, 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 0.30-0.38% by weight. steel crystallizes without the formation of bubbles and pores in an ingot or continuously cast billet. With a lower nitrogen content, the required mechanical properties are not achieved, with a higher nitrogen content, bubbles and pores may form in the ingot.

Copper gives steel of this composition increased corrosion resistance and, with the stated content of other elements, increases the upper temperature of the γ region. When the copper content of 0.6-1.4% of the mass. the equilibrium structure of γ + Cr ₂ B steel in the temperature range from 1050 to 1300 ° C, which is guaranteed to provide an austenitic structure and the required set of properties under industrial conditions. With a lower copper content, corrosion resistance in acidic environments and sea water is reduced. A higher copper content is undesirable, since the lower temperature of the austenitic region rises and the heterogeneity of the melt and, accordingly, of the finished steel in terms of chemical composition and properties is possible.

Boron with its content from more than 0.01 to 0.015% and the declared content of alloying elements forms chromium borides in the solid metal below the solidus temperature, which prevent grain growth during heating of steel in the austenitic region before hot deformation and before quenching. A lower boron content is inefficient, with a higher content of it, chromium borides are formed in a liquid metal during crystallization, have a larger size and adversely affect the properties of steel.

Aluminum in the specified range of 0.015-0.035% of the mass. 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 a permissible value of the equivalent chromium concentration Cr _e . At a higher silicon content, Cr _e increases and δ-ferrite may appear in the steel structure. 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≤0.015, 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.

In the method of thermo-deformation processing according to the invention, the steel has an austenitic basic structure and the required complex of mechanical and physical characteristics. If the heating temperatures before deformation are not observed, the beginning and end of the operations of thermal deformation processing, the degree of compression and the cooling rate for operations, obtaining austenitic steel and achieving its characteristics declared according to the invention is impossible. An example of the implementation of steelmaking and steel processing technology

In an experimental manner, the steels of the claimed composition were smelted 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, nitrided ferrochrome, ferroboron.

The obtained ingots were heated to a temperature of 1250 ° C and forged in the temperature range of 1250-1050 ° C with a degree of deformation of 70%, then the forgings were cooled in air and cleaned.

Further, the forgings were heated to 1180 ° C and rolled with a total degree of deformation of 60% (to a thickness of 10 mm) in the temperature range of 1180-1080 ° C in 9 passes with intermediate heating. After rolling, the resulting blanks were cooled in air.

The final rolling was carried out according to the scheme of high-temperature thermomechanical processing. The metal was heated to 1150 ° C and rolled onto a sheet with a total degree of deformation of 60% (to a thickness of 6 mm) in the range of 1150-1080 ° C with intermediate heating. The final cooling of the rolled products was carried out at a rate of 100 ° C / s with water. Then the rolled metal was stripped and cut to the required dimensions. The chemical composition of the steels is presented in table 1. The mechanical properties of the obtained alloys are presented in table 2.

* Tensile tests at -175 ° C, impact bending at -196 ° C

The corrosion resistance of the developed alloys in an acidic environment (0.5 M H ₂ SO ₄ , pH = 0.44) and sea water (3% NaCl) for different indicators (MCC, total, pitting and crevice corrosion) is not lower or higher than that of corrosion-resistant stainless steels of the type (05-12) X18H (8-10) and 06X18AH (8-10).

Claims (19)

1. An ingot of structural cryogenic austenitic high-strength corrosion-resistant welded steel containing carbon, chromium, nickel, manganese, molybdenum, silicon, nitrogen, aluminum, iron and impurities, in which it contains sulfur, phosphorus, tin, lead, bismuth and arsenic, characterized in that the steel 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 - 1.8-2.2 Si - 0.25-0.35 N - 0.30-0.38 Cu - 0.6-1.4 B - from more than 0.01 to 0.015 Al - 0.015-0.035 S≤0.0025 P≤0.015 Sn≤0.005 Pb≤0.005 Bi≤0.005 As≤0.005 Fe is the rest. 2. The method of thermo-deformation processing of an ingot according to claim 1, which consists in heating the ingot to a temperature of 1250-1050 ° C, forging it into a workpiece with a total degree of deformation of at least 50%, cooling the workpiece in air, heating the workpiece to a temperature of 1180 -1080 ° С followed by rolling with a total compression ratio of at least 40% and cooling in air to produce rolled products, heating the rolled products to a temperature of 1150-1080 ° C and final rolling in 2-3 passes with a total compression ratio of 30-80% and temperature the end of rolling 1050-1080 ° C, after which carry out accelerated cooling at a speed of 20-100 ° C / s to room temperature.