RU2693990C1 - Steel, article from steel and method of its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to production of martensite-austenitic grade stainless steel intended for production of high-loaded parts working for torsion and bending under dynamic load in aggressive acidic media with high content of alkali and alkali-earth metal salts, salts of nitric and sulfuric acids, chlorine ions, hydrogen sulphide. Steel contains components, wt%: carbon 0.005÷0.07; silicon not more than 1.0; manganese not more than 1.8; chromium 12.5÷17.0; nickel 2.0÷8.0; molybdenum + 3 × tungsten 0.05÷4.5; nitrogen 0.005÷0.15; boron 0.0001÷0.01; at least one of components: aluminum, titanium, niobium – 0.01÷5.0; iron and impurities – balance. At that (Mo+3·W)≤(k₁-Cr·a₁), where k₁=15.9, a₁=0.87, as well as Ni=k₂-a₂·(Cr+Mo+W), where k₂=16.25±1.5, a₂=0.7±0.1. Method of article manufacturing includes casting into ingots or continuously-cast billets, rolling and thermal treatment.

EFFECT: high ductility, corrosion resistance in hydrogen sulphide media with simultaneous improvement of stability of mechanical properties of steel.

25 cl, 2 tbl

Description

The invention relates to ferrous metallurgy, in particular to methods for manufacturing steel products, steel products, as well as the martensitic-austenitic class stainless steel itself, which is intended for the manufacture of high-loaded parts working in torsion and bending under dynamic load and in aggressive acidic environments with high content of salts of alkali and alkaline earth metals, salts of nitric and sulfuric acids, chlorine ions, hydrogen sulfide.

Known steel of the following composition, wt. %:

carbon - 0.01-0.07,

silicon - 0.4-0.8,

manganese - 0.4-0.8,

chromium - 15.0-17.0,

Nickel - 2.5-4.5,

copper - 1.6-3.0,

niobium - 0.15-0.35,

iron - the rest (see RU №2215815 S1.10.11.2003)

The prior art is also known product made of high-strength corrosion-resistant steel austenitic-martensitic class, strengthened with nitrogen, intended for the manufacture of high-loaded machine parts, in particular aircraft operating at temperatures from minus 70 ° C to 300 ° C in any climatic conditions, (see, for example, RU 2214474 C2, 10/20/2003, 7 C 22 C 38/48).

The lack of known steels, as well as products made from them, consists in insufficient plasticity, instability of the steel structure in aggressive acidic environments, as a result of which the mechanical properties of a steel product may deteriorate with time during operation.

The problem solved by the invention is the creation of steel, as well as products from it, with high ductility, optimal corrosion resistance, primarily in hydrogen sulfide environments, while simultaneously increasing the stability of the mechanical properties of steel during operation.

This problem in part of the steel is solved by the fact that the steel according to the invention contains carbon, silicon, manganese, chromium, nickel, iron, nitrogen, molybdenum, tungsten, boron, and at least one of the group: aluminum, titanium, niobium, vanadium in an amount, wt.%:

carbon 0.005 ÷ 0.07; silicon not more than 1.0; manganese not more than 1.8; chromium 12.5 ÷ 17.0; nickel 2,0 ÷ 8,0; molybdenum + 3⋅ tungsten 0.05 ÷ 4.5; nitrogen 0.005 ÷ 0.15; boron 0.0001 ÷ 0.01; at least one of the group: aluminum, titanium, niobium, vanadium 0.01 ÷ 5.0; iron and impurities rest,

provided that the content of its components satisfies the following relations (Mo + 3⋅W) ≤ (k ₁ -Cr⋅a ₁ ), where k ₁ = 15.9, a ₁ = 0.87, and also Ni = k ₂ - a ₂ ⋅ (Cr + Mo + W), where k ₂ = 16.25 ± 1.5, a ₂ = 0.7 ± 0.1.

The steel may additionally contain copper (0.05 ÷ 5.0) wt.%.

Steel may contain at least one of the following additional components: calcium, cerium, barium, rare earth metals, zirconium, yttrium, magnesium, arsenic, tantalum, selenium.

Each additional component may contain in the amount of (0.001 ÷ 0.1) wt.%.

The steel may additionally contain lanthanum in the amount of (0.005 ÷ 0.02) wt.%.

Steel may additionally contain cobalt in an amount of not more than 1.0 wt.%.

This task in terms of the method of manufacturing products from steel is solved by the fact that according to the invention, the product is obtained from the above-described steel, and the steel is poured into ingots or continuously cast billets, and then rolled to produce billets, mostly cylindrical in shape, which is subjected to heat treatment in the following modes: heating and holding the products at a temperature of (300 ÷ 650) ° C for 1 ÷ 17 hours, followed by cooling in air or in an environment with high cooling capacity, for example, water or oil.

Steel can be smelted in electric arc furnaces.

Steel before casting may be subjected to vacuum in the ladle.

Steel can be cast into ingots weighing 0.7-2.5 tons or continuously cast billets of square or round cross-section with a side of a square or with a diameter of 80 to 220 mm.

Rolling can be carried out in two stages: at the first stage on the blooming with obtaining blanks of predominantly square section, and then on a small section mill - on blanks of predominantly cylindrical shape.

The side of square billets can be from 80 to 120 mm.

The diameter of the cylindrical billets can be preferably from 12 mm to 45 mm.

After receiving blanks, mostly cylindrical, before heat treatment, the blanks can be cut into measuring rods, then the head can be transported in a cold or hot state with subsequent preliminary heat treatment, and after applying the thread using the knurling or cutting method, a product is made in the form of a bolt or screw.

Or, after receiving blanks, mainly cylindrical, before heat treatment, the blanks can be cut into measuring rods with preliminary heat treatment followed by threading by knurling or cutting to produce a product in the form of a pin.

Preliminary heat treatment can be carried out in the mode: heating to (900-1150) ° С, holding 1-100 minutes, cooling in air or in an environment with high cooling capacity, for example, water or oil.

They can perform repeated heat treatment with the modes of heating and holding products at a temperature of (300-650) ° C for 2-15 hours, followed by cooling in air or in an environment with high cooling capacity, for example, water or oil.

This task in part of the steel product is solved due to the fact that the product according to the invention is made of the above-described steel.

The product can be made mainly in the form of a cylindrical rod with a diameter of 12 to 45 mm.

The product can be made in the form of a shaft, for example, a submersible pump or gas separator with a length of up to 8.5 meters.

The roughness of its surface R _a can be no more than 2.5 microns at a base length of 0.8 mm.

The product may have a yield strength of at least 90 kgf / mm ² .

The product may have a straightness deviation of no more than 0.2 mm per linear meter of the product.

The hardness of the product can be 444 ÷ 285 HB with a print diameter of 2.9 ÷ 3.6 mm.

The product can be made in the form of a fastener, for example, a bolt, screw or stud from M5 to M20.

The technical result is steel, a method of manufacturing a product from steel, as well as a steel product itself with enhanced ductility, optimal corrosion resistance and strength in aggressive, primarily in hydrogen sulfide environments, while simultaneously increasing the stability of the mechanical properties of the steel during operation due to optimal selected ratio of steel components, as well as heat treatment regimes.

So, molybdenum and tungsten are introduced into the steel within the specified limits in order to increase the corrosion resistance, especially to pitting corrosion. In this sense, the effect of molybdenum and tungsten is equivalent. When the content of the amount of molybdenum + 3 * tungsten is less than 0.05%, the influence of these elements on the corrosion resistance does not manifest itself. Increasing the content of molybdenum + 3 * tungsten by more than 4.5% will not lead to a further increase in the corrosion resistance of the steel. Tungsten has a significantly greater atomic weight (183.85) compared with molybdenum (45.44). Clause 6 of the notes to Table 1 of GOST 4543-71 provides for the possibility of replacing elements at the rate of three weight parts of tungsten per weight part of molybdenum.

Tungsten, due to the larger size of the atom, introduces a greater distortion in the crystal lattice of iron compared to molybdenum. This allows you to increase the strength properties of steel and from this point of view, the use of tungsten is preferable.

On the other hand, tungsten and molybdenum are expensive elements, and since doping tungsten steel requires three times more than molybdenum, the use of tungsten for doping can lead to a significant increase in the cost of steel.

Aluminum, titanium, niobium, vanadium participate in the dispersion hardening of steel during its heat treatment due to the release of intermetallic compounds such as Ni ₃ Me. Since the action of these elements is considered equivalent, therefore, when doping steel, one of a group of elements or all elements at the same time can be applied.

When the content of these elements is less than 0.02%, there is no additional hardening of the steel due to precipitation hardening.

With an increase in the content of these elements, the strength characteristics of the steel increase, but at the same time the ductility and toughness of steel decrease.

When the content of these elements is more than 5%, the ductility of steel and the toughness become low, which can lead to breakdowns of finished products during operation.

Molybdenum, tungsten and chromium are ferrite-forming elements. With simultaneous alloying of steel Mo, W, Cr at the upper limit of their content, steel can go into a ferrite-austenitic class, instead of a martensitic-austenitic class.

The ratio (Mo + 3W) ≤ (k ₁ -Cr / a ₁ ) limits the upper limit of the content of Mo and W depending on the amount of chromium introduced. This eliminates the transition of steel in the austenitic-ferritic class.

The second formula Ni = k ₂ -a ₂ (Cr + Mo + W) establishes the connection between the austenitic-forming element Ni and the ferrite-forming elements Cr, Mo, W. The fulfillment of the conditions of the formula also ensures the production of martensitic-austenitic steel.

The coefficient k ₂ has an interval of values k _2min = 14.75 and k _2max = 17.75. If the value of nickel is lower than the steel calculated at k _{2min, the} steel acquires martensitic or martensitic-ferritic structure with reduced plastic properties.

When the nickel content is more than calculated at k _{2max, the} steel acquires an austenin-martensitic structure with an austenite content of more than 30%. As a result, the strength properties of steel are reduced.

Thus, the nickel content in steel depends on the number of ferrite-forming elements and is determined by the formula N _i = k ₂ -a ₂ (Cr + Mo + W), where a ₂ is a correction factor.

High strength of steel can be ensured with the martensitic-ferritic steel structure, as, for example, in steel according to patent RU 2215815. In this case, with the same content of chromium (ferrite-forming elements), less nickel content (austenitic elements) is required.

At the same time, nickel is a highly plastic, corrosion-resistant element. Increasing the nickel content in steel, we give it greater ductility, which can be characterized by the following parameters: relative elongation, relative narrowing, impact strength, steel resistance to cyclic fatigue, etc., as well as improving corrosion resistance, primarily in hydrogen sulfide environments.

Equal strength is due to approximately equal content of martensite. In the first case, ferrite is contained as an excess phase; in the second, austenite.

Carbon in steel can form chromium carbides, which in the case of a carbon content of more than 0.07%, significantly deteriorate the ductility of the steel and the toughness. It is necessary to strive for a minimum carbon content. At the same time, carbon production of less than 0.005% is associated with technical difficulties and material costs due to the use of special methods of steelmaking and the use of highly pure charge materials. At the same time, a reduction in the carbon content of less than 0.005% will not lead to a significant improvement in the plastic properties of the steel.

Silicon and manganese in our case are technological additives used to deoxidize steel. Their content in steel to 1.0 wt.% And to 1.8 wt.%, Respectively, do not affect the service properties of steel. Higher content may lead to deterioration of plastic properties.

When the chromium content is less than 12.5%, the corrosion resistance of steel deteriorates sharply. When the chromium content is more than 17%, an additional phase is formed in the steel - ferrite. As a result, the strength properties of steel are reduced, and the ductility is deteriorated.

Nitrogen is introduced into steel for the purpose of additional hardening of steel, primarily due to the formation of nitrides of niobium, titanium, and vanadium.

Fine particles of metal nitrides are evenly distributed throughout the grain volume, additionally reinforced steel. When the nitrogen content is less than 0.005%, additional hardening due to the formation of metal nitrides will be negligible. When the nitrogen content is more than 0.15%, along with significant hardening of steel, a decrease in plastic properties will be observed.

The introduction of boron in steel leads to an improvement of plastic properties, first of all, toughness. The selection of borides of metals at the grain boundaries prevents the release of harmful elements of sulfur, phosphorus at the grain boundaries.

Doping with boron less than 0.0001 will not provide a noticeable improvement in plastic properties. At the same time, when doping with boron in an amount of more than 0.01, as a result of the formation of an excessive amount of metal borides, a decrease in plastic properties begins.

The required level of mechanical properties of steel products is ensured by the specified heat treatment modes.

The properties of the dispersion-hardening steel are determined by the amount and dispersion of the precipitated intermetallic particles. At a temperature of less than 300 ° C, the processes proceed slowly. The initial stresses of martensite are not sufficiently removed. As a result, the steel does not acquire the required strength, and because of the unheated martensite in the future during the operation of the products, their breakage is possible.

At temperatures close to, but exceeding 300 ° C, due to the slow course of the processes, considerable exposures up to 17 hours are required in order to obtain noticeable hardening.

With increasing temperature, the intensity of the formation of intermetallic particles increases, the steel becomes more durable. The maximum strength of steel reaches at 450-500 ° С. At higher temperatures, the intermetallic compounds stand out in larger sizes. At the same time, the achievable strength of steel is reduced, and the minimum required strength of steel is achieved at a temperature not exceeding 650 ° C.

With a shutter speed of less than 1 hour, the amount of intermetallic particles will be insufficient for appreciable hardening of the steel at any temperatures.

With a shutter speed of more than 17 hours, the growth of precipitated intermetallic particles occurs, resulting in a decrease in the strength of steel.

Thus, the proposed steel, a product made from it, as well as a method of manufacturing a product from steel, provide enhanced ductility while maintaining high strength, their stability during operation, as well as resistance to stress corrosion cracking when working in corrosive environments.

Example.

Steel was smelted in the main electric arc furnace. Casting of steel was carried out into ingots of 1.15 tons. The ingots were rolled on a blooming on a billet square 100 mm. The billets were rolled on a small section mill to rods with a diameter of 20 mm and a length of 5400 mm. Heat treatment of rods was double tempering in the following modes:

- heating and holding the bars at a temperature of 600 ° C for 4 hours, followed by cooling in air;

- re-heating and aging of the bars at a temperature of 600 ° C for 4 hours, followed by cooling in air.

Mechanical properties were determined on the finished bars.

Testing of mechanical properties was carried out according to GOST 1497-43, impact strength according to GOST 9454-78.

The resistance of steel to stress corrosion cracking in a hydrogen sulfide medium was carried out according to the procedure of the NACE TM 0177-96 standard (USA). The sample was placed in the medium of an aqueous solution of hydrogen sulfide and a tensile force was applied to it, which created a stress in the metal equal to 70% of the yield strength of the steel. The resistance of steel to stress corrosion cracking in hydrogen sulfide was determined as the time elapsed from the beginning of the test to the complete destruction of the sample. The chemical composition of steel produced with different content of components, the results of tests of mechanical properties and corrosion tests are given in Tables 1, 2.

Table 1 Chemical composition of steel Option Chem. composition one 2 3 four five 6 carbon 0.015 0.07 0,005 0.015 0.015 0.015 silicon 0.3 1.0 0.3 0.3 0.3 0.3 manganese 0.4 0.4 1.0 0.4 0.4 0.4 chromium 15 15 15 15 15 17 nickel* fact five 2.4 7.2 1.9 6.8 3 payment 2,1 ÷ 6,1 2.4 ÷ 5.4 4,2 ÷ 7,2 2,1 ÷ 6,1 2,1 ÷ 6,1 2,0 ÷ 5,0 molybdenum 1.5 2.55 0.02 1.5 1.5 1.2 tungsten 0.1 0.1 0.01 0.1 0.1 0.1 molybdenum + 3⋅ tungsten * fact 1.8 2.85 0.05 1.8 1.8 1.5 payment ≤2.85 ≤2.85 ≤2.85 ≤2.85 ≤2.85 ≤1.1 nitrogen 0.05 0.15 0,005 0.05 0.05 0.05 boron 0.001 0,005 0.01 0,008 0.001 0.0001 aluminum + titanium + niobium + vanadium 3 0.02 5.0 3 3 3 *) In table No. 1 for the indicated components, besides the actual the contents of the components in the steel composition options are given calculated required by the formula of their values.

table 2 Mechanical and corrosion properties of steel Option Properties one 2 3 four five 6 Limit 125 120 128 125 100 115 flowability kg / mm ² Temporary 130 127 132 130 110 120 resistance breaking kg / mm ² Relative 60 58 60 50 60 55 narrowing,% Relative 20 18 20 12 20 17 elongation,% Shock 120 110 120 80 120 100 viscosity, J / cm ² Corrosive 800 750 800 700 800 760 stamina have become under by stress hour

Options 1, 2, 3 consistent with the invention. Option 1 is optimal. Options 4, 5 - do not satisfy this invention, as they have a nickel content beyond the boundaries determined by the formula. As a result, option 4 has lower values of ductility and corrosion resistance, since steel has become pure martensitic, and version 5 has reduced strength properties due to excessive austenite content.

Option 6 has a molybdenum content of + 3 * tungsten more defined by the formula. Steel has a high content of ferrite and residual austenite and, as a result, reduced strength and plastic characteristics.

Claims (29)

1. Steel, characterized by the fact that it contains carbon, silicon, manganese, chromium, nickel, iron, nitrogen, molybdenum, tungsten, boron and at least one of the group: aluminum, titanium, niobium in an amount, wt.%: Carbon 0,005-0,07 Silicon no more than 1.0 Manganese no more than 1.8 Chromium 12.5-17.0 Nickel 2.0-8.0 Molybdenum + 3⋅ tungsten 0.05-4.5 Nitrogen 0.005-0.15 Boron 0.0001-0.01 At least one of the group: aluminum, titanium, niobium 0.01-5.0 Iron and impurities rest, provided that the content of its components satisfies the following relationships: (Mo + 3⋅W) ≤ (k ₁ -Cr⋅a ₁ ), where k ₁ = 15.9, a ₁ = 0.87, and Ni = k ₂ -a ₂ ⋅ (Cr + Mo + W), where k ₂ = 16.25 ± 1.5, and ₂ = 0.7 ± 0.1. 2. The steel according to claim 1, characterized in that it additionally contains copper 0.05-5.0 wt.%. 3. The steel according to claim 1, characterized in that it contains at least one of the following additional components: calcium, cerium, barium, rare earth metals, zirconium, yttrium, magnesium, arsenic, tantalum, selenium. 4. The steel according to claim 3, characterized in that each additional component is contained in an amount of 0.001-0.1 wt.%. 5. The steel according to claim 1, characterized in that it additionally contains lanthanum in an amount of 0.005-0.02 wt.%. 6. The steel according to claim 1, characterized in that it additionally contains cobalt in an amount of not more than 1.0 wt.%. 7. A method of manufacturing a product made of steel, characterized in that the product is produced from steel according to any one of claims 1 to 6, and the steel is poured into ingots or continuously cast billets, after which rolling is carried out to obtain billets of predominantly cylindrical shape, which is subjected to heat treatment in the following modes: heating and holding the products at a temperature of 300-650 ° C for 1-17 hours, followed by cooling in air or in an environment with increased cooling capacity, such as water or oil. 8. The method according to claim 7, characterized in that the steel is smelted in an electric arc furnace. 9. The method according to p. 7, characterized in that the steel before casting is subjected to vacuum in the ladle. 10. The method according to p. 7, characterized in that the steel is poured into ingots weighing 0.7-2.5 tons or continuously cast billets of square or circular cross section with a side of a square or with a diameter of 80 to 220 mm. 11. The method according to claim 7, characterized in that the rolling is carried out in two stages: in the first stage, blooming is performed to produce blanks of predominantly square cross-section, and then, on a small section mill, into blanks of predominantly cylindrical shape. 12. The method according to claim 11, characterized in that the side of the square-section blanks is from 80 to 120 mm. 13. The method according to claim 11, characterized in that the diameter of the cylindrical billets is preferably from 12 to 45 mm. 14. The method according to claim 7, characterized in that after obtaining blanks of predominantly cylindrical shape, the blanks are cut into measuring rods, then the head is disembarked in a cold or hot state, followed by preliminary heat treatment, and after applying the thread using the knurling or cutting method bolt or screw. 15. The method according to claim 7, characterized in that after receiving blanks of predominantly cylindrical shape, the blanks are cut into measuring rods, then the measuring rods are subjected to preliminary heat treatment followed by threading by knurling or cutting to obtain a product in the form of a hairpin. 16. The method according to any of paragraphs.14 and 15, characterized in that the preliminary heat treatment is carried out in the mode: heating to 900-1150 ° C, holding 1-100 min, cooling in air or in an environment with high cooling capacity, such as water or oil 17. The method according to claim 7, characterized in that they reheat with the modes of heating and holding the products at a temperature of 300-650 ° C for 2-17 hours, followed by cooling in air or in an environment with enhanced cooling capacity, such as water or oil 18. A product of steel, characterized in that it is made of steel according to any one of claims 1 to 6. 19. The product according to claim 18, characterized in that it is made mainly in the form of a rod of a cylindrical shape with a diameter of from 12 to 45 mm. 20. The product according to claim 18, characterized in that it is made in the form of a shaft, for example a submersible pump or gas separator with a length of up to 8.5 m. 21. The product according to claim 18, characterized in that its surface roughness R _{a is} not more than 2.5 μm at a base length of 0.8 mm. 22. The product according to claim 18, characterized in that it has a yield strength of at least 90 kgf / mm ² . 23. The product according to claim 18, characterized in that it has a straightness deviation of not more than 0.2 mm per linear meter of the product. 24. The product according to claim 18, characterized in that its hardness is 444-285 HB with a print diameter of 2.9-3.6 mm. 25. The product according to claim 18, characterized in that it is made in the form of a fastener, for example a bolt, screw or stud in the size from M5 to M20.