RU2406780C2 - Stainless steel produced by duplex process - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: invention refers to stainless steel produced by duplex process. Steel is designed for fabrication of elements of structures for installations generating power and producing materials in chemical, oil-chemical and papermaking industries Steel contains the following elements, wt %: C ëñ 0.05, 21 ëñ Cr ëñ 25, 1 ëñ Ni ëñ 2.95, 0.16 ëñ N ëñ 0.28, Mn ëñ 2.0, Mo + W/2 ëñ 0.50, Mo ëñ 0.45, W ëñ 0.15, Si ëñ 1.4, Al ëñ 0.05, 0.11 ëñ Cu ëñ 0.50, S ëñ 0.010, P ëñ 0.040, Co ëñ 0.5, P3M ëñ 0.1, V ëñ 0.5, Ti ëñ 0.1, Nb ëñ 0.3, Mg ëñ 0.1, the rest - iron and additives at melting; also microstructure of steel consists of austenite and 35-65 vol. % of ferrite. Composition of steel corresponds to dependencies: 40 ëñ IF ëñ 70, where: IF=6ù(% Cr + 1.32ù% Mo + 1.72ù% Si) - 10ù(% Ni + 24ù % C + 16.15ù% Nù0.5ù% Cu + 0.4ù% Mn) - 6.17 and IRCL ëÑ 30.05 where: IRCL=% Cr + 3.3ù% Mo + 16ù%N + 2.6ù% Ni - 0.7ù% Mn. Sheets, strips, bands, rods, wires, profiles, forged pieces and casts are made out of this steel. ^ EFFECT: improved properties of steel. ^ 28 cl, 5 dwg, 7 tbl, 1 ex

Description

The present invention relates to stainless steel obtained by the duplex process, in particular to steel intended for the manufacture of structural elements for plants for the production of materials in the chemical and petrochemical industry and paper production, etc. or energy generation, as well as to a method of manufacturing sheets, strips , ingots, wires or profiles of this steel.

In a broader sense, this steel can be used instead of 304L stainless steel in many areas, for example, in the above industries or in the food industry, including the manufacture of parts from molded wire (welded grilles, etc.), profiles (sprinklers, etc.), axes, etc. Castings and forgings can also be made.

For this purpose, it is known to use stainless steels of grades 304 and 304L, the microstructure of which after annealing is essentially austenitic; in the state after hardening, they may additionally contain martensite in different quantities. However, these steels contain significant nickel additives, the cost of which is usually high. In addition, these steel grades can cause problems from a technical point of view when used in some areas, since in the annealed state they have low tensile properties, in particular a low elastic limit, as well as low resistance to stress corrosion.

Stainless austenitic-ferritic steels are also known, consisting, as a rule, of a mixture of ferrite and austenite, they are 1.4362; 1.4655; 1.4477; 1.4462; 1.4507; 1.4410; 1.4501 and 1.4424 according to the EP 10088 standard, all of them contain nickel in an amount of over 3.5%. These steels are particularly corrosion resistant and are also resistant to stress corrosion.

Also known are stainless steels, the so-called ferritic or ferrite-martensitic steels, the microstructure of which for a given range of heat treatments consists of two phases: ferrite and martensite, preferably in a ratio of 50:50, for example, steel 1.4017 according to EN 10088. When the amount of chromium is in quantity as a rule, less than 20% of these steels have high mechanical tensile properties, but are characterized by poor corrosion resistance.

In addition, it is also necessary to simplify the method of manufacturing steel sheets, strips, rods, wire and profiles.

The purpose of the present invention is to eliminate the disadvantages inherent in the prior art steels and methods for their production, and to create stainless steel with high mechanical properties, in particular with a tensile strength of more than 400, even 450 MPa, in the annealed state or after transfer to a solution with high corrosion resistance greater than or equal, in particular, corrosion resistance of 304L steel, high microstructure resistance and high toughness in welded areas without the need for additives in steel of expensive alloying elements, as well as in creating an easily feasible method for the production of sheets, strips, rods, wire and profiles from this steel.

The first object of the invention is stainless steel of the following composition, obtained by the duplex process, wt.%:

C≤0.05

21≤Cr≤25

1≤Ni≤2.95

0.16≤N≤0.28

Mn≤2.0

Mo + W / 2≤0.50

Mo≤0.45

W≤0.15

Si≤l, 4

0.11≤Cu≤0.50

S≤0.010

P≤0.040

Co≤0.5

REM ≤0.1

V≤0.5

Ti≤0.1

Nb≤0.3

Mg≤0.1,

the rest is iron and impurities formed during smelting, while the microstructure of the steel consists of austenite and 35-65 vol.% ferrite, and the composition corresponds to the following dependencies:

40≤IF≤70, preferably 40≤IF≤60,

where IF = 6 × (% Cr + 1.32 ×% Mo + 1.27 ×% Si) -10 × (% Ni + 24 ×% C + 16.15 ×% N × 0.5 ×% Cu + 0 , 4 ×% Mn) -6.17,

and

IRCL≥30.05, preferably ≥32,

where IRCL =% Cr + 3.3 ×% Mo + 16 ×% N + 2.6 ×% Ni-0.7 ×% Mn.

The steel according to the invention may also have the following optional properties, taken separately or in combination:

- the amount of ferrite is from 35 to 55 vol.%,

- the chromium content is from 22 to 24 wt.%,

- the manganese content is less than 1.5 wt.%,

- the calcium content is less than 0.03 wt.%,

- the molybdenum content is more than 0.1 wt.%.

The second object of the invention is a method of manufacturing a hot-rolled sheet, strip or strip of steel according to the invention, including:

- preparing an ingot or slab from steel having a composition according to the invention,

- hot rolling the specified ingot or slab at a temperature of from 1150 to 1280 ° C to obtain a sheet, strip or tape.

According to a particular case of the method, said ingot or slab is hot rolled at a temperature of 1150 to 1280 ° C. until a so-called plate is obtained, heat treatment is carried out at a temperature of 900 to 1100 ° C. and the resulting plate is cooled by quenching in air.

A third object of the invention is a method for producing a hot rolled steel bar or wire according to the invention, in which:

- make an ingot or bloom obtained by continuous casting of steel having a composition according to the invention,

- roll the specified ingot or bloom in a hot state at a temperature of from 1150 to 1280 ° C and get a bar, cooled in air, or a coil of wire, cooled in water, and optionally:

- conduct heat treatment at a temperature of from 900 to 1100 ° C and

- cool the specified bar or coil wire quenching.

According to a particular case of the method after cooling, it is additionally possible to produce a cold draw of the specified bar or wire drawing.

The invention also relates to a method for manufacturing a steel profile in which cold profiling of a hot-rolled bar obtained according to the invention is carried out, as well as to a method for manufacturing a steel forgings in which the hot-rolled bar obtained according to the invention is divided into blanks and the workpiece is forged at a temperature of 1100 to 1280 ° C.

In addition, the invention relates to various products obtained by the methods according to the invention, and to their use, such as:

- hot-rolled steel plates from 5 to 100 mm, strips and strips for the production of structural elements for plants for the production of materials or energy, in particular for plants for the production of materials and energy, operated at a temperature of from -100 to 300 ° C, preferably from -50 up to 300 ° C;

- steel strips obtained by cold rolling of a hot rolled strip;

- hot rolled bars with a diameter of 18 to 250 mm and cold rolled bars with a diameter of 4 to 60 mm, suitable for the production of machine parts, such as parts for pumps, axles for valves, axles for motors and fittings for corrosive environments;

- hot rolled wire with a diameter of 4 to 30 mm and a drawn wire with a diameter of from 0.010 to 20 mm, which can be used in the cold stamping of prefabricated products intended for the food industry, for the extraction of oil and ores, or for the production of metal sheets and wickerwork for chemical filtration substances, ores or substances in the food industry;

- profiles;

- forgings for the production of flanges or fittings;

- castings obtained by casting steel according to the invention.

Other features and advantages of the invention are set forth below in the description, given by way of example only.

The stainless steel obtained by the duplex process according to the invention contains components, the contents of which are given below.

The carbon content of this steel is not more than 0.05 wt.%, Preferably less than 0.03 wt.%. Indeed, too high a content of this element reduces the resistance to local corrosion and increases the risk of precipitation of chromium carbides in the areas of heat exposure during welding.

The chromium content in the steel is from 21 to 25 wt.%, Preferably from 22 to 24 wt.%, Which is necessary to achieve good corrosion resistance, which would be at least equivalent to the corrosion resistance of steel grades 304 or 304L.

The nickel content in this steel is from 1 to 2.95 wt.%, Preferably not more than 2.7 wt.% Or even 2.5 wt.%. This austenite-forming element is introduced in order to obtain positive properties that provide resistance to the formation of corrosion shells. When the content is more than 1 wt.%, Preferably more than 1.2 wt.%, It has a beneficial effect, preventing the formation of pitting corrosion. However, its content is limited, since when the content is more than 2.95 wt.%, There is a decrease in resistance to the spread of the mentioned pitting corrosion. The addition of this element also allows for an optimal compromise combination of impact strength and ductility. Indeed, it is of interest to shift the curve of the phase transition of impact strength to the low-temperature region, which is especially preferred in the production of thick plates, for which the impact strength values are of great importance.

Since the nickel content in the steel according to the invention is limited, it was found that for the formation of an appropriate amount of austenite after heat treatment at 900-1100 ° C, it is necessary to introduce other elements forming austenite in an unusually large amount and to limit the content of elements forming ferrite.

The nitrogen content in the steel is from 0.16 to 0.28%, which, as a rule, makes it necessary to introduce nitrogen into the steel during its smelting. This austenite-forming element makes it possible, first of all, to obtain steel with two phases ferrite + austenite by the duplex process, while the amount of austenite provides high resistance to stress corrosion, and also allows to obtain increased mechanical properties of the metal. Nitrogen also makes it possible to ensure high stability of the microstructure in the heat-affected area by welding. Its maximum content is limited, since with a content of over 0.28%, solubility difficulties arise: shells form during the hardening of slabs, blooms, ingots, castings or during welding.

The content of manganese, which also forms austenite at temperatures below 1150 ° C, is maintained at a level of less than 2.0 wt.%, Preferably less than 1.5 wt.%, Due to the adverse effects of this element in many respects. For example, difficulties arise in the smelting and refining of steel, since this element corrodes some types of refractory materials in ladles, which makes it necessary to more frequently replace these expensive elements and, therefore, more frequent interruptions of the process. Ferromanganese additives, usually introduced to refine the steel composition, additionally contain significant amounts of phosphorus and selenium, the introduction of which into steel is undesirable and which are difficult to remove when refining steel. In addition, manganese impairs refining and limits the possibility of decarburization. It also causes difficulties in subsequent processing, as causes the steel to corrode due to the formation of manganese sulfides MnS and oxide inclusions. This element was traditionally introduced into steel, in which it was required to increase the nitrogen content to increase the solubility of this element in steel. In the absence of a sufficient amount of manganese, it becomes impossible to provide the necessary nitrogen content. However, the authors of the present invention were able to establish that it is possible to limit the addition of manganese to the steel according to the invention, while ensuring the introduction of a sufficient amount of nitrogen, and to achieve the desired effect on the ferritic-austenitic equilibrium in the base metal, as well as to ensure the stability of the sections after heat treatment by welding.

Molybdenum, which is a ferrite-forming element, is maintained in an amount of less than 0.45 wt.%, And the tungsten content in an amount of less than 0.15 wt.%. However, the content of both of these elements must be such that their sum Mo + W / 2 is less than 0.50 wt.%, Preferably less than 0.4 wt.%, Particularly preferably less than 0.3 wt.%. Indeed, the inventors have found that when the content of both of these elements and their sum is less than the above values, there were no cases of intermetallic embrittlement, which allows, in particular, to simplify the method of production of steel sheets or strips, providing the possibility of their cooling in air after heat treatment or application hot. In addition, it was found that while ensuring the content of elements in their required limits, the weldability of steel increases. However, it is preferable to maintain a minimum molybdenum content of 0.1% to increase the ductility of the steel in the hot state. In addition, the smelting of steel with a molybdenum content of less than 0.1% implies a significant limitation on the amount of reusable metal scrap for this steel, which causes difficulties in application, making it necessary, in particular, to use a cage consisting of 100% ferroalloys only.

Copper, which is an austenite-forming element, is present in an amount of from 0.11 to 0.50 wt.%, Preferably from 0.15 to 0.40 wt.%. This element improves corrosion resistance in a reducing acidic environment. However, its content is limited to 0.50 wt.% To prevent the formation of epsilon phases, since they harden the ferrite phase and embrittle the alloy obtained by the duplex process.

The oxygen content is preferably limited to 0.010 wt.% To increase the ductility.

Boron is an optional element that can be introduced into the steel according to the invention in an amount of from 0.0005 to 0.01 wt.%, Preferably from 0.0005 to 0.005 wt.%, Particularly preferably from 0.0005 to 0.003 wt.%, to improve hot forming. According to another embodiment, it is preferable to limit the boron content to less than 0.0005 wt.% To limit the risk of cracking during welding and continuous casting.

Silicon, which forms a ferrite element, is contained in an amount of less than 1.4 wt.%. Aluminum, which is a ferrite forming element, is present in an amount of less than 0.05 wt.%, Preferably from 0.005 to 0.040 wt.%, Which is necessary for the formation of inclusions of calcium aluminates with a low melting point. The maximum aluminum content is also limited to prevent excessive formation of aluminum nitrides. The purpose of both of these elements, silicon and aluminum, is mainly to ensure a high degree of deoxidation of liquid steel during its smelting.

Cobalt, which is a ferrite forming element, is maintained in an amount of less than 0.5 wt.%, Preferably less than 0.3 wt.%. This element is the residual introduced by the starting materials. In particular, its contents are limited due to difficulties in transporting parts after irradiation in nuclear installations.

Rare earth metals (REM) can be introduced into the composition of the steel in an amount of 0.1 wt.%, Preferably less than 0.06 wt.%. Especially worth mentioning is cerium and lanthanum. The content of these elements is limited due to their ability to form undesirable intermetallic compounds.

Vanadium, which is a ferrite-forming element, can be introduced into steel in an amount of 0.5 wt.%, Preferably less than 0.2 wt.%, To increase its resistance to peptic corrosion.

Niobium, which is a ferrite forming element, can be added to steel in an amount of 0.3 wt.%, Preferably less than 0.050 wt.%. It allows to improve the mechanical tensile strength of steel due to the formation of small niobium nitrides in it. To limit the formation of large niobium nitrides, its content is limited.

Titanium, which is a ferrite-forming element, can be added to steel in an amount of 0.1 wt.%, Preferably less than 0.02 wt.%, To limit the formation of titanium nitrides, in particular in liquid steel.

Also, in the steel according to the invention, calcium can be added in an amount of less than 0.03 wt.%, Preferably more than 0.0002 wt.%, Even more preferably more than 0.0005 wt.%, To control the type of oxide inclusions and to improve workability. The content of this element is limited due to the fact that, together with sulfur, it is able to form calcium sulfides, which reduce corrosion resistance. According to a preferred embodiment, the calcium content is limited to less than 0.0005 wt.%, Preferably less than 0.0002 wt.%.

The sulfur content is maintained at a level of less than 0.010 wt.%, Preferably less than 0.003 wt.%. As already shown above, this element forms, together with manganese or calcium sulfides, the presence of which negatively affects the corrosion resistance. Sulfur is considered an impurity.

The addition of magnesium in an amount not exceeding a final content of 0.1 wt.% Can be made to change the type of sulfides and oxides.

Selenium is preferably contained in an amount of less than 0.005 wt.% Due to its negative effect on corrosion resistance. This element is introduced into steel, as a rule, in the form of impurities contained in ferromanganese ingots.

Phosphorus is maintained in an amount of less than 0.040 wt.% And is considered an impurity.

The remaining composition of the steel is iron and impurities. In addition to the above elements, zirconium, tin, arsenic, lead and bismuth should also be indicated. Tin may be present in an amount of less than 0.100 wt.%, Preferably less than 0.030 wt.%, To prevent welding complications. Arsenic may be contained in an amount of less than 0.030 wt.%, Preferably less than 0.020 wt.%. Lead may be contained in an amount of less than 0.002 wt.%, Preferably less than 0.0010 wt.%. Bismuth may be contained in an amount of less than 0.0002 wt.%, Preferably less than 0.00005 wt.%. Zirconium may be contained in an amount of not more than 0.02 wt.%.

In addition, the inventors found that in the case when the content of chromium, molybdenum, nitrogen, nickel and manganese in weight percent satisfies the dependence below, the corresponding steels are highly resistant to local corrosion, i.e. to the formation of pinholes or sinks:

IRCL =% Cr + 3.3 ×% Mn + 16 ×% N + 2.6 ×% Ni-0.7 ×% Mn≥30.5.

The microstructure of the steel according to the invention in the annealed state is formed by austenite and ferrite, and the amount of ferrite after treatment for 1 h at 1000 ° C is from 35 to 65 vol.%, Preferably from 35 to 55 vol.%.

The inventors of the present invention also found that the following formula appropriately transfers the ferrite content at 1100 ° C:

IF = 6 × (% Cr + 1.32 ×% Mo + 1.27 ×% Si) -10 × (% Ni + 24 ×% C + 16.15 ×% N + 0.5 ×% Cu + 0, 4 ×% Mn) -6.17.

Therefore, to ensure ferrite in an amount of from 35 to 65% at a temperature of 1100 ° C, the IF index should be from 40 to 70.

In the annealed state, the microstructure does not contain any other phases that could adversely affect the mechanical properties, in particular, such as the sigma phase and other intermetallic phases. In the caged state, part of the austenite can transform into martensite, which depends on the actual temperature of the deformation and the degree of cold deformation.

Typically, the steel according to the invention can be smelted and used in the form of hot rolled sheets, also called plates, as well as hot rolled strips obtained from slabs or ingots, and in the form of cold rolled strips obtained from hot rolled strips. Hot rolling may also be carried out for the manufacture of rods or wire rods or profiles or forgings; then these products can be subjected to hot working in the form of forging or cold working to obtain rods or profiles by drawing or wire drawing. The steel according to the invention can also be used to produce castings, followed by or without heat treatment.

To achieve the best possible results, it is preferable to apply the method according to the invention, which includes, first of all, the manufacture of an ingot, slab or bloom from steel, the composition of which corresponds to the invention.

An ingot, slab or bloom is obtained, as a rule, by melting the starting materials in an electric furnace, followed by melting in a vacuum furnace such as AOD or VOD to ensure decarburization. After that, steel can be cast to produce ingots, slabs or blooms by continuous casting in a mold without a bottom. It is also possible to cast steel directly in the form of thin slabs, in particular by continuous casting between oppositely rotating rolls.

After preparation of the ingot, slab or bloom, if necessary, re-heating to a temperature of 1150-1280 ° C is carried out, however, it is also possible to directly process the slab coming from the continuous casting unit using casting heat.

In the manufacture of sheets, a slab or ingot is rolled in a hot state and a plate is obtained, the thickness of which is usually from 5 to 100 mm. The degree of compression used at this stage ranges, as a rule, from 3 to 30%. This plate is then subjected to heat treatment to transfer into the solution the precipitates formed at this stage due to heating to a temperature of 900-1100 ° C, and then the plate is cooled.

The method according to the invention provides cooling by quenching in air, which is easier to carry out than traditional cooling used for such a steel grade, which is faster cooling with water. However, if necessary, it is also possible to apply water cooling.

Such slow cooling in air is possible, in particular, due to the limited content of nickel and molybdenum in the composition of the steel according to the invention, in which no intermetallic phases are released that adversely affect the technological properties. The specified cooling can be carried out, in particular, at speeds from 0.1 to 2.7 ° C / s.

At the end of hot rolling, the plate can be straightened, cut and etched if it is provided in such final form.

You can also roll such unprotected steel on a tape mill to a thickness of 3 to 10 mm.

In the manufacture of long products from ingots or blooms, hot rolling can be carried out with a single heating on a multi-stand mill with calibrated rolls at a temperature of 1150 to 1280 ° C to obtain wire or wire rod from bars. The ratio between the sections of the initial bloom and the final product is preferably more than 3, which allows for the internal defect-freeness of the rolled product.

After the manufacture of the bar, it is cooled at the exit of the rolling mill by air blowing.

In the case when the wire rod is made with a diameter of more than 13 mm, it can be cooled at the exit of the rolling mill in a tank with water.

In the manufacture of a wire with a diameter of less than or equal to 13 mm, it can be cooled by immersing individual turns in water after they pass on a conveyor in 2-5 minutes through an oven for transferring into a solution at a temperature of 850 to 1100 ° C.

Subsequent heat treatment in the furnace at a temperature of 900-1100 ° C can optionally be carried out for rods or coils of wire that have already passed the heating stage during rolling, in the case when it is necessary to complete the crystallization of the structure and slightly reduce the mechanical tensile strength.

After cooling the rods or coils of wire, different types of processing are carried out for hot or cold forming, depending on the purpose of the product. Therefore, after cooling, it is possible to carry out a cold drawing of the rods or wire drawing.

It is also possible cold profiling of hot rolled bars or the manufacture of parts after cutting the bars into billets and forging them.

To clarify the invention, the tests described below were carried out, in particular with reference to the drawings, in which:

figure 1 shows the relationship between the percentage of ferrite after treatment at 1100 ° C and the IF index for the starting materials;

figure 2 shows the relative diametrical deviation of the delta ⌀ depending on the deformation temperature;

figure 3 illustrates the potentials of the point holes E1 and E2, which were determined on the forged bars taking into account the indicator IRCL;

figure 4 illustrates the rate V of uniform corrosion, which was determined on the forged bars taking into account the IRCL;

figure 5 shows the critical temperature of the CST and CPT, which was determined on the forged bars taking into account the indicator IRCL.

Examples

Laboratory ingots weighing 25 kg were obtained by melting starting materials and pure ferroalloys in an induction vacuum furnace, nitrogen was introduced by adding nitrided ferroalloys at a partial nitrogen pressure and poured into a metal mold with an external nitrogen pressure of 0.8 bar. Of these ingots, only experimental ingots 1441 and 14604 were in accordance with the invention.

150 tons of reference steel of grade 8768 were used for industrial casting according to the invention. This steel was smelted in an electric furnace, followed by its refinement and decarburization in a vacuum so that the required carbon level could be achieved. Then it was fed to a continuous casting unit to obtain slabs with a cross section of 220 × 1700 mm, which were rolled in a hot state after heating to 1200 ° C for the manufacture of plates with a thickness of 7, 12 and 20 mm. The resulting plates were then subjected to heat treatment at a temperature of about 1000 ° C to transfer the various precipitates formed at this stage into the solution. After heat treatment, the plates were cooled in water, subjected to dressing, cutting and etching.

The compositions of different steels smelted in the laboratory and in industrial conditions, in weight percent, are given in table 1, and also table 1 for comparison shows the compositions of various industrial products and semi-finished products smelted in an electric furnace, refined by AOD, cast in ingots or in a continuous way.

1. Ferrite content

1.1 the content of ferrite in the starting materials

On lumpy samples with a volume of 1 to 3 cm ³ cut from laboratory castings that were unprocessed after casting or from industrial unprocessed castings after casting, in a salt bath with immersion in water after processing, heat treatment was carried out for 30 minutes at different temperatures in order to determine the amount of ferrite at high temperature. Since, in contrast to austenite, ferrite has magnetic properties and was probably contained in carbides and nitrides, the dosing method was applied by measuring saturated magnetization. The ferrite content, which was determined in this case, is shown in table 2 and shown in figure 1.

When considering figure 1, we can note a good agreement between the IF index and the measured ferrite content in the base metal after treatment at 1100 ° C.

In addition, in casting 14441 at a temperature below 1300 ° C, the ferrite content corresponds to the hot deformation of the double grain size structure. After processing this casting in the range of 950-1100 ° C, the ferrite content in it was sufficient to provide resistance to stress corrosion.

Air force - blooms obtained by continuous casting.

1.2 Ferrite content in final products

The ferrite content was measured by a grid method (according to ASTM E 562) on forged bars after heat treatment at 1030 ° C and in the heat-affected zones when welding seams with coated electrodes at constant energy with a cooling rate of 20 ° C / s at 700 ° C. The results (ferrite content in the base metal and in the heat-affected zone) are shown in Table 3. It can be concluded that castings 14441 and 14604 according to the invention contain ferrite in the base metal and in the heat-affected zone, which increases resistance to local and stress corrosion, and impact strength (see table 5).

Table 3 Ferrite content Ref Product α M.V.,% α Z.A.T.,% 14441 * Forged bar 48 70 14604 * Forged bar 54 65 14382 Forged bar 49 80 14383 Forged bar 79 88 14660 Forged bar 48 72 UNS S32101 Hot rolled sheet 45 67 UNS S32304 Hot rolled sheet 47 75 * According to the invention

α MV,% - ferrite content measured in the base metal.

α Z.A.T.,% - ferrite content measured in the heat-affected zone.

2. Foundry properties

Ingot 14439 contained shells and was unusable. To eliminate this phenomenon when casting in air at atmospheric pressure, it was necessary to limit the nitrogen content in the castings according to the invention to less than 0.28 wt.%.

3. The ability to hot deformation

The ability to hot deformation was evaluated using hot tensile tests of samples, the calibrated part of which with a diameter of 8 mm and a length of 5 mm was heated under the action of the Joule effect for 80 seconds to 1280 ° C, then cooled at a speed of 2 ° C / s to the test temperature, which varied from 900 to 1280 ° C. When this temperature was reached, a quick extension was immediately carried out at a speed of 73 mm / s; after rupture, the narrowed neck diameter was measured at the rupture site.

The relative diametrical deviation (table 4), given below, gives an idea of the ability to hot deformation:

Delta ⌀ = 100 × (1 - (final diameter / initial diameter)).

Table 4 Relative diametric deviations (tensile tests in the hot state) Test temperature, ° С Delta ⌀,% Casting 14382 Casting 14383 Casting 14441 * 1280 85.0 100.0 96.7 1250 98.3 86.7 1200 75.0 98.3 76.7 1150 70.0 95.0 61.7 1100 63.3 93.3 56.7 1050 51.7 75.0 44,2 1010 45.0 1000 65.0 40,0 980 36.7 960 58.3 950 35.8 900 35.0 51.7 36.7 * According to the invention

From table 4 and figure 2, in which the data are presented in the form of curves, it follows that the casting 14441 according to the invention has the ability to hot deformation, which can be compared with the same deformation of the reference comparative casting 14382.

4. Mechanical properties

The tensile strength Re _0.2 and R _{m were} determined according to NFEN 10002-1. The impact strength KV was determined at different temperatures according to NFEN 10045.

Table 5 Mechanical properties Ref Product Re _0.2 , MPa R _m , MPa KV 20 ° C, J KV -50 ° C, J 14441 * Forged bar 477 716 334 51 14604 * Forged bar 477 691 288 eighteen 14382 Forged bar 436 664 > 339 339 14383 Forged bar 458 604 79 9 14660 Forged bar 493 701 293 31 304L Hot rolled sheet 218 523 312 301 316L Hot rolled sheet 232 537 307 298 UNS S32101 Hot rolled sheet 466 720 101 60 UNS S32304 Hot rolled sheet 438 663 268 153 8768 * Hot rolled sheet 519 743 * According to the invention

Re _0.2 is the elastic limit during deformation of 0.2%.

R _m is the tensile strength.

The results obtained on laboratory castings 14441 and 14604, as well as on industrial casting 8768, which are obtained according to the invention, indicate that an elastic limit of more than 450 MPa can be achieved, i.e. twice the elastic limit of austenitic steels of the AISI 304L type.

The values of impact strength at 20 ° C obtained for laboratory castings 14441 and 14604 and industrial castings 8768, which are obtained according to the invention, exceed 200 J, which is satisfactory, taking into account the elastic limit of these steels. For castings 14383, not related to the invention, with a low nitrogen content and a high ferrite content in the annealed state, the impact strength at 20 ° C. was less than 100 J. This confirms the need to introduce a sufficient amount of nitrogen to provide a satisfactory viscosity level.

5. Corrosion resistance

At the same time, corrosion tests were carried out on forged bars from laboratory furnaces and on samples cut from hot rolled sheets made from industrial castings.

5.1 Resistance to local corrosion

Resistance to pitting corrosion was evaluated by plotting potential intensity curves and determining the potential of a punch hole at i = 100 μA / cm ² . This parameter was measured in a neutral (pH 6.4), highly chlorinated ([Cl -] = 30 g / l) medium at 50 ° C (E ₁ ), typical of brines in seawater desalination plants, and slightly acidified (pH 5.5), slightly chlorinated ([Сl -] = 250 ppm) at an ambient temperature (E ₂ ) typical of drinking water. The critical temperature for point holes in an iron-containing chlorinated medium (FeCl ₃ , 6%) was also measured in accordance with ASTM G48-00, method C.

In another series of tests, the resistance to pitting corrosion in a neutral deaerated medium was determined at a NaCl content of 0.86 mol / L, which corresponds to 5 wt.% NaCl at 35 ° C. The aging potential was measured for 900 s. Then a potential-dynamic curve was constructed at a speed of 100 mV / min from the moment of exposure to the potential of a point hole. The potential of the pinhole (E ₃ ) was determined at i = 100 μA / cm ² . Under these conditions, samples of steel according to the invention were tested, as well as control samples of steel grade 304L and duplex process austenitic-ferritic steels of type 1.4362 and others.

The resistance to peptic corrosion was studied by measuring the critical temperature of the shell in a neutral (pH 6.4), highly chlorinated ([Cl -] = 30 g / l) medium. The unit for measuring ulcerative corrosion corresponded to the recommendations contained in ASTM G78-99. The critical temperature of the shell is the minimum temperature at which shells with a depth of more than 25 microns are observed.

The values obtained are shown in Table 6. A comparison of the results obtained on a sheet of UNS S32304 steel and on a bar made of casting 14382, which have a similar chemical composition, shows that the corrosion resistance of the bar is lower than the corrosion resistance of a hot-rolled sheet of the same composition.

The authors of the present invention, it was found that the coefficient of resistance to local corrosion, i.e. the formation of pinholes and sinks, expressed by the abbreviation IRCL and defined as:

IRCL = Cr + 3.3 × Mo + 16 × N + 2.6 × Ni-0.7 × Mn

(content of Cr, Mo, N, Ni, Mn in wt.%),

convincingly shows the resistance of the entire complex of compositions with a nickel content of less than 6% against local corrosion (see Fig.3, 4, 5). Castings 14383 and 14660, not related to the invention, the IRCL of which is 28.7 and 29.8, have worse corrosion resistance compared to steel type AISI 304L. Castings 14604 and 14441 according to the invention, the IRCL of which is 30.9 and 33, have at least the same properties as type 304L steel. It has been found that in order to achieve a corrosion resistance which is at least equal to that of AISI 304L steel, it is necessary that the steels according to the invention preferably have an IRCL of more than 30.5, more preferably more than 32.

5.2 Resistance to uniform corrosion

Uniform corrosion was determined by evaluating the corrosion rate through weight loss after immersion for 72 hours in a 2% solution of sulfuric acid heated to 40 ° C.

Comparison of corrosion rates in pilot castings with a content of 2.5% Ni and 0.2% N (14441 according to the invention and 14660, which is not related to the invention) also showed the negative effect of a high manganese content on the resistance to uniform corrosion in a sulfuric acid environment.

Table 6 Local and uniform corrosion resistance data Ref Product IRCL E ₁ (V / ECS) E ₂ (V / ECS) E ₃ (V / ECS) CPT, ° C SST, ° C V, mm / year 14441 * Forged bar 33.0 0.165 1,058 0.320 7.5 fifty 0.73 14604 * Forged bar 30.9 0.159 0.802 5 45 1.8 14382 Forged bar 35.8 0.302 1,323 0.420 fifteen 60 0.24 14383 Forged bar 28.7 0,049 0.595 0,050 0 35 4.95 14660 Forged bar 29.8 0,094 0.707 7.5 45 1,11 304L Hot rolled sheet Not suitable 0.188 0.834 0.210 5 65 316L Hot rolled sheet Not suitable 0.266 0.865 7.5 75 UNS S32101 Hot rolled sheet 26,4 0.163 0.855 12.5 UNS S32304 Hot rolled sheet 35.7 0.413 1,330 ¹ 17.5 95 517077 Rolled bar 34.6 0.415 140301 Rolled bar 47.1 1,200 ¹ 8768 * Hot rolled sheet 33.1 0.227 1,273 ¹ * according to the invention;
¹ oxidative potential of solvent, no point holes noted

E ₁ is the potential of a point hole in a neutral (pH 6.4), highly chlorinated medium (30 g / l Cl ') at 50 ° C;

E ₂ is the potential of a point hole in slightly acidified (pH 5.5), slightly chlorinated medium (250 ppm Cl ') at 25 ° C;

E ₃ - the potential of a point hole in a neutral chlorinated medium (NaCl 5%) at 35 ° C;

CPT is the critical temperature of a point hole in a chlorinated, iron-containing medium;

CCT is the critical temperature of the shell in a neutral (pH 6.4), highly chlorinated (30 g / l Cl ') medium;

V - uniform corrosion rate in a 2% sulfate medium at 40 ° C.

5.3 The potential for repassivation

Steel samples were ground in water with SiC paper to a value of 1200, then aged in air for one day.

The cyclic polarization test in chlorinated water was carried out by starting with the measurement of the aging potential for 15 minutes followed by cyclic dynamic polarization at 100 mV / min, starting from the aging potential to the potential at which the electric current reaches a force of 300 μA / cm ² and returns to the potential at which it is equal to zero.

Thus, the values of the potentials of the point hole (Vpit) and the potentials of repassivation (V-repassivation) of the previously formed point holes were determined. The results are presented in table 7.

Table 7 V-passivation depending on nickel content Casting % Ni Vpit - V-passivation, mV / ECS 14382 4,5 460 14441 2.5 361 14383 1,5 227

In accordance with tests of repassivation potentials in a NaCl medium, it was found that the higher the nickel content, the greater the gap between the potential of a pinhole and the repassivation potential, which indicates the adverse effect of nickel on the repassion of steel according to the invention subjected to earlier pitting.

Claims (28)

1. Stainless steel obtained by the duplex process, having the composition, wt.%:
C≤0.05
21≤Cr≤25
1≤Ni≤2.95
0.16≤N≤0.28
Mn≤2.0
Mo + W / 2≤0.50
Mo≤0.45
W≤0.15
Si≤1,4
Al≤0.05
0.11≤Cu≤0.50
S≤0.010
P≤0.040
Co≤0.5
REM ≤0.1
V≤0.5
Ti≤1
Nb≤0.3
Mg≤0.1
the rest is iron and impurities formed during smelting, while the microstructure of the steel consists of austenite and 35-65 vol.% ferrite, and the composition also corresponds to the following dependencies:
40≤IF≤70,
where IF = 6 · (% Cr + 1.32 ·% Mo + 1.27 ·% Si) -10 · (% Ni + 24 ·% С + 16.15 ·% N · 0.5 ·% Cu + 0 4% Mn) -6.17
and IRCL≥30.05,
where IRCL =% Cr + 3.3 ·% Mo + 16 ·% N + 2.6 ·% Ni-0.7 ·% Mn. 2. Steel according to claim 1, characterized in that IRCL≥32. 3. Steel according to claim 1 or 2, characterized in that the amount of ferrite is from 35 to 55 vol.%. 4. The steel according to claim 1, characterized in that 40≤IF≤60. 5. Steel according to claim 1, characterized in that the chromium content in it is from 22 to 24 wt.%. 6. Steel according to claim 1, characterized in that the manganese content in it is less than 1.5 wt.%. 7. Steel according to claim 1, characterized in that the calcium content in it is less than 0.03 wt.%. 8. Steel according to claim 1, characterized in that the molybdenum content in it is more than 0.1 wt.%. 9. A method of obtaining a hot-rolled steel sheet, strip or strip of steel having a composition according to any one of claims 1 to 8, in which:
make an ingot or slab of steel,
the specified ingot or slab is rolled in a hot state at a temperature of from 1150 to 1280 ° C to obtain a sheet, strip or tape. 10. The production method according to claim 9, in which:
the specified ingot or slab is rolled in a hot state at a temperature of from 1150 to 1280 ° C to obtain a sheet, then
conduct heat treatment at a temperature of from 900 to 1100 ° C and
cool the specified sheet by rapid cooling in air. 11. A hot-rolled steel sheet obtained by the method of claim 10 and having a thickness of 5 to 100 mm. 12. The use of hot-rolled steel sheet, strip or strip obtained by the method according to claim 9 or 10, for the manufacture of structural elements for plants for the production of material or energy generation. 13. The use according to claim 12, wherein said plants for the production of material and energy are operated at a temperature of from -100 to 300 ° C, preferably from -50 to 300 ° C. 14. Steel cold rolled strip obtained by cold rolling of a hot rolled strip made by the method according to claim 9. 15. A method of obtaining a steel bar or wire of steel having a composition according to any one of claims 1 to 8, in which:
prepare an ingot or bloom by continuous casting of steel,
hot rolled ingot or bloom at a temperature of from 1150 to 1280 ° C to obtain a bar, cooled in air, or a coil of wire, cooled in water, and optional
conduct heat treatment at a temperature of 900-1100 ° C and
cool the bar or bay by rapid cooling. 16. The rod obtained by the method according to clause 15 and having a diameter of from 18 to 250 mm 17. The wire obtained by the method according to clause 15 and having a diameter of from 4 to 30 mm 18. The method according to clause 15, in which carry out a cold drawing rod or wire drawing after cooling. 19. The rod obtained by the method according to p. 18 and having a diameter of from 4 to 60 mm 20. The wire obtained by the method according to p. 18 and having a diameter of from 0.010 to 20 mm 21. The use of the rod according to clause 16 or 19 for the production of machine parts, such as parts for pumps, axles for valves, axles for engines and fittings for aggressive environments. 22. The use of wire according to claim 17 or 20 for the cold forming of composite products for the food industry, oil and ore production, for the production of metal sheets and fabrics for filtering chemicals, ore or substances obtained in the food industry. 23. A method of manufacturing a steel profile in which cold profiling of a hot-rolled bar obtained by the method according to clause 15 is performed. 24. Steel profile obtained by the method according to item 23. 25. A method of manufacturing a steel forgings, in which the hot-rolled bar obtained by the method according to item 15, is divided into blanks and subjected to the specified blank forging at a temperature of from 1100 to 1280 ° C. 26. Steel forgings obtained by the method of claim 25. 27. The use of forgings according to p. 26 for the manufacture of flanges and fittings. 28. The casting obtained by casting steel according to any one of claims 1 to 8.

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