RU2584315C1 - Structural cryogenic austenite high-strength corrosion-resistant, including bioactive media, welded steel and method of processing - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention is intended for making cryogenic high-strength welded structures used in transportation of liquefied gases. Steel contains, in wt.%: C - 0.05-0.07, Cr - 18.0-20.0, Ni - 5.0-7.0, Μn - 9.0-11.0, Mo - 1.4-1.8, Si - 0.25-0.35, Cu - 2.0-2.2, Ν - 0.28-0.32, Al - 0.015-0.035, S≤0.0025, Ρ≤0.010, Sn≤0.005, Pb≤0.005, Bi≤0.005, As≤0.005, Fe is rest. Nitrogen and copper content are connected by ratio: Ν×Cu=0.610-0.650. Steel is obtained by heating ingot, deformation of ingot into billet with total deformation degree of 40-90 % in temperature range of 1,250-1,100 °C, cooling workpiece at air, cleaning, rolling obtained workpiece at temperature of 1,200-1,080 °C with total reduction ratio of 45-70 %, final rolling for 2-3 passes with total reduction ratio of 30-80 % at temperature of 1,150-1,080 °C to produce rolled metal and its further accelerated cooling at rate of 20-100 °C/s.

EFFECT: steel has high mechanical properties at room temperature, required viscosity in field of cryogenic temperatures and good weldability.

2 cl, 1 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.

The austenitic steel according to the patent RU 2102522 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 and second objects of the proposed invention is a corrosion-resistant high-strength non-magnetic steel and the method of its production and processing (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 it contains sulfur as 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. %

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.

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.

The disadvantages of this steel and its production method are as follows.

This steel crystallizes with the formation of γ and δ phases. With a significant number of combinations of the basic elements of Cr, Ni, Mn, V, Nb, Mo, 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 in excess nitrogen will be released into the gas phase and form bubbles and porosity in the ingot. In addition, the simultaneous production of V, Nb, Al, Ca, Mg, Na in steel in the stated proportions 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 of these elements and their relationship.

The disadvantages of the technology of thermal deformation processing according to patent RU 2392348 C2, publ. 06/20/2010 are excessive detailing of operations that impedes the implementation and control of the technology, in addition, with the recommended heating mode before deformation, for some element ratios, the steel structure when heated ≥ 1200 ° C 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 present invention, a technical result is achieved, which consists in obtaining structural cryogenic austenitic high-strength corrosion-resistant, including in bioactive environments, welded steel and the method for its preparation and processing, suitable for various industries, including for the manufacture of cryogenic high-strength welded structures, used in the transportation of liquefied gases, with the following characteristics:

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

- viscosity in the region of cryogenic temperatures, KCU ^{(-163) ° С} ≥34 J / cm ² ;

- good weldability;

- profitability, as it has a low nickel content;

- stable austenitic structure in the high-temperature region, excluding the formation of δ-ferrite;

- corrosion resistance in an acidic environment and sea water;

- high anti-adhesive stability against corrosive microorganisms (sulfate-reducing bacteria);

- manufacturability due to the fact that with a relatively low manganese content, the required nitrogen content can be obtained by smelting at normal pressure in existing units.

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

Structural cryogenic austenitic high-strength corrosion-resistant, including in bioactive environments, weldable 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 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 - 2.0-2.2,

N - 0.28-0.32,

Al - 0.015-0.035,

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 nitrogen and copper contents are related by the ratio N × Cu = 0.610-0.650.

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

The method of thermal deformation processing of structural cryogenic austenitic high-strength corrosion-resistant, including in bioactive media, steel includes heating the ingot, deformation of the ingot into the workpiece with a total degree of deformation of 40-90% in the temperature range of 1250-1100 ° C, cooling the workpiece in air and cleaning rolling the resulting billet at temperatures of 1200-1080 ° C with a total degree of compression of 45-70% and final rolling in 2-3 passes with a total degree of compression of 30-80% at temperatures of 1150-1080 ° C to obtain a rolled product that costs m is subjected to cooling at a rate of 20-100 ° C / sec.

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 and the ratio of the copper and nitrogen contents from the condition N × Cu = 0.610-0.650, the equilibrium structure of the steel is lower than the solidus temperature and up to 1080 ° C consists of a γ-phase, which is guaranteed to provide in real technological conditions a purely austenitic structure and the required set of properties.

The proposed steel is also characterized by high efficiency, as it has 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, which, together with nitrogen, provides 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 and molybdenum within specified limits with a copper content of 2.0-2.2 wt.% And nitrogen 0.28-0.32 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, corrosion resistance in acidic medium and sea water, high anti-adhesive resistance to corrosive microorganisms (sulfate reducing bacteria) and suitability for the manufacture of cold-resistant high fusion welded structures used in the transportation of liquefied gases and petroleum products.

When the content of alloying elements (Cr, Ni, Mn, Mo) below the claimed limit, it is impossible to achieve a purely 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 makes it difficult to dissolve the 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 range defined by the invention, and the nitrogen content of 0.28-0.32 wt.% 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 resistance in bioactive environments. With a copper content of 2.0-2.2 wt.% And a ratio of copper and nitrogen contents N × Cu = 0.610-0.650, the equilibrium structure of the steel below the solidus temperature and to 1080 ° C consists of a γ phase, which is guaranteed to ensure production in industrial conditions a purely austenitic structure and the required set of properties. At a lower copper content in the steel structure below the solidus temperature, the δ phase appears and the corrosion resistance in bioactive media decreases. A high copper content is undesirable, since the heterogeneity of the melt and, accordingly, the finished steel in chemical composition and properties is possible.

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 chromium oxides are possible, a higher aluminum content leads to the formation of high-temperature aluminum nitrides and makes it difficult to obtain a purely austenitic structure.

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 . At a higher silicon content, Cr _e increases and 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≤0.010, 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.

In the method of thermal deformation processing according to the invention, the steel has a purely austenitic structure and the required complex of mechanical and physical characteristics. If the heating temperatures before deformation, the beginning and end of the operations of thermal deformation processing, the degree of compression and the cooling rate for operations are not observed, it is impossible to obtain pure austenitic steel and achieve its characteristics declared according to the invention.

An example of the implementation of steelmaking and steel processing technology

Experimentally, the steel of the claimed composition was smelted in a vacuum induction furnace with a capacity of 50 kg for liquid metal using the above-described features of the technology for smelting nitrogen-containing corrosion-resistant steels. Pure charge materials were used: Armco iron, electrolytic nickel, metallic chromium and manganese, nitrided ferrochrome.

After stripping, the obtained ingot was heated to a temperature of 1250 ° C and forged in the temperature range of 1250-1100 ° C with a degree of deformation of 75%, then the forgings were cooled in air and cleaned.

Further, the forgings were rolled with a total degree of deformation of 70% (to a thickness of 10.5 mm) in the temperature range of 1200-1080 ° C for 9 passes (partial reduction of 20-30%) 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 deformed in 3 passes with a total degree of deformation of 70% (to a thickness of 3.2 mm, partial reduction of 25-30%) 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 mechanical properties of the obtained metal are presented in the table.

The corrosion resistance of the developed alloy in an acidic environment (0.5 M H ₂ SO ₄ , pH 0.44) and sea water (3% NaCl) according to various 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 06X18AN (8-10).

The steel obtained in microbiological media enriched with hydrogen sulfide and favorable for the propagation of sulfate reducing bacteria has a release ability at the level of 05Kh14N7D2AMBT and 05Kh14N9D2AMBT steels and is significantly higher than that of 07Kh14N5D2MBT steel without nitrogen. The number of adhered microorganisms (cells / cm ² ) on the samples of the obtained steel under comparable test conditions is 20 times less than for steel 07X14N5D2MBT.

Claims (2)

1. Structural cryogenic austenitic high-strength corrosion-resistant, including in bioactive environments, weldable 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 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 - 2.0-2.2
N - 0.28-0.32
Al - 0.015-0.035
S≤0.0025
P≤0.010
Sn≤0.005
Pb≤0.005
Bi≤0.005
As≤0.005
Fe - the rest,
wherein the nitrogen and copper content is related by the ratio N × Cu = 0.610-0.650. 2. The method of thermal deformation processing of structural cryogenic austenitic high-strength corrosion-resistant, including in bioactive media, welded steel according to claim 1, including heating the ingot, deformation of the ingot in the workpiece with a total degree of deformation of 40-90% in the temperature range of 1250-1100 ° C, cooling the workpiece in air and stripping, rolling the obtained workpiece at temperatures of 1200-1080 ° C with a total degree of compression of 45-70% and final rolling in 2-3 passes with a total degree of compression of 30-80% at temperatures of 1150-1080 ° C upon receipt m rolled, which is then subjected to cooling at a rate of 20-100 ° C / sec.