RU2346074C2 - Stainless high-strength steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention concerns metallurgy field and can be used at manufacturing of products from stainless high-strength steel of martensite - austenite class long-exposure at intensive dynamic loads in corrosion-active aqueous medium with presence of solved hydrogen sulfide, chlorine ions, salt of alkaline-earth metals, oil products. Steel contains carbon, chrome, nickel, copper, titanium, aluminium, molybdenum, niobium, cobalt, nitrogen, calcium, boron, cerium, silicon, manganese, sulfur, phosphor, iron and accompanying admixtures, at following ratio of components, wt %: carbon ≤ 0.03, chrome from 8.0 till less than 15.0, nickel 4.0 - 12.0, copper 0.3 - 5.0, titanium 0.01 - 1.0, aluminium 0.01 - 0.6, molybdenum 0.05 - 3.0, niobium 0.05 - 0.4, cobalt 0.01 - 3.0, nitrogen 0.005 - 0.15, calcium ≤ 0.02, boron ≤ 0.005, cerium ≤ 0.02, silicon ≤ 0.8, manganese ≤ 0.8, sulfur ≤ 0.03, phosphor ≤ 0.03, iron and accompanying admixtures are the rest. Nickel and chromic equivalents are associated by following ratio: Cr_equiv / Ni_equiv (1.7 and Ni_equiv - (22.3 ± 3)-0.83Cr_equiv.

EFFECT: providing of high values of yield strength and impact elasticity, and also increased corrosive attack strength under tension.

5 cl, 3 tbl, 1 ex

Description

The invention relates to metallurgy and can be used in the manufacture of stainless steel products of martensitic-austenitic class, working for a long time under intense dynamic loads in a corrosive aqueous medium with the presence of dissolved hydrogen sulfide, chlorine ions, alkaline earth metal salts, oil products, for example, parts of submersible equipment in oil production.

Known corrosion-resistant high-strength steel composition (wt.%): C 0,08-0,12; Cr 13.5-14.5; Ni 3.5-4.5; N, 0.15-0.2; Mo 1-1.5; Mn 1-1.5; V 0.03-0.05; Nb 0.03-0.05; Si 0.1-0.3; Cu 0.3-0.8; Ca 0.01-0.03; Ba 0.01-0.03; Fe - the rest (see patent RU 2214474 C2, (1)).

Also known is the “Steel Product” in the form of a bar made of steel of martensitic-austenitic class, hardened by carbides and carbonitrides of tungsten, vanadium, molybdenum, niobium, intermetallic compounds of the type Ni ₃ Me composition (wt.%): C 0.005-0.07; Si <1.0; Mn <1.8; Cr 12.5-17.0; Ni 2.0-8.0; Mo + 3W 0.05-4.5; N, 0.005-0.15; B 0.0001-0.01; at least one of the group: AL, Ti, Nb, V 0.01-5.0; Fe and impurities - the rest (see patent RU 45998 U1, (2)).

Known austenitic stainless steel according to patent EP 1645649, containing carbon, chromium, nickel, copper, titanium, aluminum, molybdenum, niobium, cobalt, nitrogen, calcium, boron, cerium, silicon, manganese, iron and related impurities, in the following ratio of components, wt.%:

C ≤ 0.1 Ti 0.001-1.0 Ce 0.0001-0.2 Si ≤ 1,0 Al ≤ 0.1 S ≤ 0.03 Mn 0.01-30.0 Co 0.3-5.0 P ≤ 0.03 Cr 15.0-30.0 Nb 0.05-0.4 B 0.0001-0.02 Ni 5.0-30.0 Ca 0.0001-0.005 Cu 0.3-2.0 Mo 0.3-3.0 N, 0.001-0.3 Fe - the rest

The disadvantage of products from known high-strength stainless steels is not sufficiently high complex of mechanical properties that is actually achievable in them, namely, the combination of yield strength with impact strength. So in steel (2) there is a large number of elements that allow hardening during aging or tempering (Fe, Al, Mo, V, B, Nb). However, it is not possible to really obtain high hardening in the product due to the strong embrittlement of steel. A significant amount of nickel, which determines the toughness of high-strength steel, is bound into intermetallic compounds of the type Ni ₃ (Ti, Al, Mo), and the impact strength is therefore low. By heat treatment, a satisfactory value of impact strength can be achieved, but at a relatively low level of strength. It should also be mentioned the relatively low resistance to corrosion under stress under the influence of dissolved hydrogen sulfide, chlorine ions, salt solution due to the lack of copper in the steel and low resistance to brittle fracture due to the low impact strength.

The technical problem solved by the invention is the production of stainless steel with a tensile strength in the range from 1078 to 1470 MPa with an impact strength of KCU _{+20 of} at least 68.6 J / cm ² and increased resistance to stress corrosion.

This problem is solved in that the composition of the steel, in addition to Fe, C, Cr, Ni and related impurities, contains one or more elements from the group - Cu, Ti, Al, Mo, Nb, Co, N, Ca, B, Ce in an amount (wt. .%):

C ≤ 0.03 Ti 0.01-1.0 Ce ≤ 0.02 Si ≤ 0.8 Al 0.01-0.6 S ≤ 0.03 Mn ≤ 0.8 From 0.01-3.0 P ≤ 0.03 Cr from 8.0 to less than 15.0 B ≤ 0.005 Nb 0.05-0.4 Ni 4.0-12.0 Cu 0.3-5.0 Ca ≤ 0.02 Mo 0.05-3.0 N, 0.005-0.15 Fe - the rest

with the ratio of ferrite-forming and austenite-forming elements with their equivalence coefficients

The steel production process may include electric arc smelting with vacuum-oxygen refining in the ladle, hot rolling and heat treatment, at the temperature of the end of hot rolling, the temperature is limited in the range from 970 to 1050 ° C, and heat treatment can consist of several modes in the temperature range from -70 ° C to 950 ° C with a holding time of at least 2 hours. In addition, in a preferred, but not required embodiment of the invention, the steel may have a tensile strength in the range of 1078 to 1470 MPa, impact strength of at least 68.6 J / cm ² and pitting corrosion equivalent> 15.

To obtain high values of yield strength and impact strength, the choice of structure and the corresponding optimal alloying of steel are important. The claimed stainless steel belongs to the martensitic-austenitic class (M + A) and in the structural diagram occupies an area with the ratio

Cr _eq. / Ni _equiv. <1.7, where the amount of Ni _equiv. = 22.3 ± 3-0.83 Cr _equiv.

In this expression, 22.3 ± 3 corresponds to the amount of Ni _equiv in steel _. (in%) in the absence of Cr _equiv. , and the coefficient of 0.83 at Cr _equiv. is the tangent of the angle of inclination to the abscissa axis of the diagram lines bounding the region of (M + A) steels.

The martensitic-austenitic structure of steel corresponds to the highest viscosity and strength of steel, because Martensite plates in steel with a carbon content of less than 0.03% turn out to be surrounded by thin viscous layers of austenite, which inhibits the development of germ cracks. With a ratio of Cr _equiv. / Ni _equiv. > 1.7 in steel, ferrite appears in larger quantities, the greater the value of this ratio. The presence of ferrite in the martensitic or (M + A) structure leads to a decrease in strength characteristics and an increase in the critical temperature of brittleness, which degrades the toughness of steel and the resistance of steel to brittle fracture under pulsed loads.

The martensitic structure, especially with the release of dispersed particles of intermetallic or excess phases during aging of steel, leads to high strength. To harden martensite with dispersed intermetallic particles, Ti, Al, Mo are present in steel, which form Ni ₃ (Ti, Al, Mo) phases with Ni. When the amount of Ti, Al is less than 0.01% and Mo is less than 0.05%, there is no effect of these elements on hardening. The upper limits of the Ti content are 1%; Al - 0.6%; Mo - 3.0% correspond to the atomic concentration of these elements in the intermetallic phases, which bind the amount of nickel in the solid solution that is acceptable for maintaining the required viscosity. So when the amount of Ti is 0.8%, the amount of bound nickel is Ni _bond. = 3.67 · 0.8 = 2.94%, which is sufficient to harden the steel and maintain a satisfactory toughness when the amount of Ni in the steel is more than 4%. But already at 1% Ti, all the nickel in this steel will be bound into intermetallic compounds, which will negatively affect the impact strength. Therefore, the Ti content is more than 1.0%, Al more than 0.6% is excessive, because binds a significant amount of Ni and leads to a decrease in toughness. The presence of Ti and Al in steel, in addition, provides a fine-grained structure, because during the crystallization of steel, Al binds nitrogen into stable nitrides, and Ti forms stable TiC carbides uniformly distributed in the grain volume and protects the steel from intergranular corrosion. A Ti content of less than 0.01% is not sufficient to bind carbon in steel.

The carbon content of the steel for the formation of martensite high viscosity should be as low as possible. The upper limit of the amount of carbon 0.03% is determined by the technological possibility of steelmaking.

To reduce the tendency to intergranular corrosion, Nb is additionally introduced into the steel, which binds carbon, forming dispersed NbC particles located in the grain volume. This delays the formation of chromium carbides at grain boundaries and the depletion of chromium solid solution at the boundaries. As follows from the atomic ratio of the elements in the NbC compound, it is necessary for the binding of 0.03% C: 0.03 · 7.73 = 0.23% Nb, therefore, the introduction of niobium in an amount of up to 0.4% completely binds carbon in steel. When the Nb content is less than 0.05%, its effect is practically not felt.

Nickel is the main element that determines the viscosity of steel and forms the austenitic component of the structure depending on the number of ferrite-forming and austenite-forming elements according to the ratio of Ni _equiv. = 22.3 ± 3-0.83 Cr _equiv . When the amount of Ni is less than 4% at a ratio of Cr _eq. / Ni _equiv. <1.7, a martensitic structure is obtained with a small amount of residual austenite, with high strength but not high viscosity. When the amount of Ni is more than 12%, the ratio of Cr _equiv. / Ni _equiv. <1.7 an austenitic structure is formed with high viscosity but low strength.

Steel for parts of submersible equipment must withstand stress damage in an active corrosive environment. In the presence of dissolved hydrogen sulfide and chlorine ions, resistance to pitting is especially important, for which steel should have a high equivalent pitting corrosion resistance (ECC), determined by the composition of the steel, namely ECC = Cr + 3.3 Mo + 16 N. It is believed that with ECC = 10 steel has a satisfactory corrosion resistance. With a minimum amount of Cr and N and a maximum amount of Mo, the inventive steel has an ESTC> 15. With an increase in the amount of Cr and N, pitting resistance increases. The sum (Cr + 3.3 Mo + 16 N) is determined by the threshold value of the electrode potential corresponding to 12.5% Cr in the absence of Mo and N, which implies that while maintaining the (M + A) structure and ECC> 15, the minimum amount Cr in steel is 8%.

With a minimum amount of Cr and a minimum amount of nickel, the steel has a martensitic structure with a small amount of residual austenite, high strength and insufficient viscosity. The upper limit of the amount of Cr is assumed to be less than 15.0%, exceeding which with a minimum amount of Ni, ferrite appears in the steel structure with a corresponding decrease in strength. The appearance of ferrite in the structure is extremely undesirable, because It is at its borders that a brittle σ phase can form and the steel's resistance to brittle fracture decreases. With a maximum Cr content and an increase in the amount of Ni up to a maximum, the structure becomes austenitic with high viscosity and low strength.

Molybdenum, like Cr, promotes the appearance of a passivating film that protects steel from corrosion, especially in the presence of Cu, and is involved in the formation of intermetallic hardening phases with Ni and Co. When the amount of Mo is less than 0.05%, its effect is practically absent. The upper limit of 3% is due to the high cost of molybdenum.

In addition to the anti-corrosion effect, copper forms dispersed particles of an excessive hardening phase during aging, and the fields of elastic stresses during their formation contribute to a large dispersion of intermetallic phases of the type Ni ₃ Me. With a lower limit of Cu content of 0.3%, its effect is practically absent. The upper limit of 5% corresponds to its maximum solubility in austenite.

The presence of Co in the composition of steel contributes to its significant hardening, as Co forms Mo and Cr dispersed phases with Mo and Cr. At the same time, Co promotes an increase in interatomic bonding forces and a decrease in the dissociation of carbide and other phases, which positively affects the resistance of steel in corrosive environments with the presence of dissolved hydrogen sulfide. The lower limit of the Co content of 0.01% corresponds to the absence of its noticeable effect. The upper limit of 3% is due to a significant rise in price of steel.

The amount of S and P is maintained in the steel at the lowest technologically achievable level - less than 0.03%, to ensure the minimum embrittlement associated with these impurities.

The presence of Si and Mn of up to 0.8% ensures sufficient deoxidation of the steel. When the content of Si and Mn is more than 0.8%, the ductility of steel deteriorates.

The introduction of nitrogen as an austenite-forming element allows to increase the amount of austenite in the structure. A nitrogen content of less than 0.005% does not affect the structure formation. The introduction of nitrogen more than 0.15% leads to a deterioration in the ductility of steel.

The introduction of calcium into steel in an amount of up to 0.02% of the calculation provides the formation of globular sulfur compounds to improve machinability and toughness.

The introduction of Ce to 0.02% of the calculation contributes to a more uniform distribution of non-metallic inclusions, improve ductility and viscosity.

The introduction of boron up to 0.005% of the calculation helps to remove sulfur and phosphorus from the grain boundaries with a beneficial effect on impact strength and corrosion resistance.

Typically, the temperature of the end of hot rolling is in the range of 900-1150 ° C. For products of the inventive steel, the temperature of the end of hot rolling, it is advisable to limit in the range of 970-1050 ° C, corresponding to the temperature interval between the beginning and end of dynamic recrystallization of austenite. The end of rolling in this temperature range contributes to the formation of a fine-grained structure and hardening of steel. In this case, conditions are created for the formation of an ordered cellular dislocation structure of austenite with an increased dislocation density in the cell walls and internal microvolumes of the cells relatively free from dislocation. The presence of Nb, Al, N in steel contributes to the fixation of the cellular structure formed at the end of hot deformation and prevents its decomposition during cooling of the declared steel products before martensitic transformation begins. In this case, the martensitic transformation occurs mainly in the cell walls, while retaining the viscous austenite between the formed martensitic plates. This structure provides an additional increase in viscosity with high strength. The end of hot deformation below a temperature of 970 ° C retains, after cooling, austenite grains elongated during hot deformation with an increased density of disordered dislocations and contributes to the hardening of steel with a decrease in viscosity. The end of hot deformation above 1050 ° C leads to coarsening of austenitic grain upon cooling of products from the declared steel due to collective recrystallization processes with a decrease in the strength and toughness of steel.

The turning of products from the inventive steel, it is advisable to produce when the hardness of steel HB is not more than 287-311, i.e. when the yield strength of steel is not more than 120 kG / mm ² . In this case, after the rental of products, heat treatment can be performed for a given strength. If it is necessary to obtain a turned product or parts from it with a yield strength of up to 150 kGf / mm ^{2, the} turning of products must be carried out without heat treatment after rolling, when the steel has an (M + A) structure with unstrengthened martensite and hardness that allows cutting to be performed. The heat treatment of products in this case is carried out after machining. Heat treatment both after hire and after machining consists of several modes in the range from minus 70 ° C to 950 ° C. Normalization from 950 ° C is carried out to improve the uniformity of the structure after rolling and increase the viscosity. Annealing at 720–760 ° С serves to stabilize residual austenite and increase the amount of martensite to increase strength and toughness. For the same purpose, cooling to minus 70 ° C with an exposure of 2 hours can be carried out. Tempering at a temperature of 500-580 ° C with a holding time of at least 2 hours is carried out to harden the steel with the release of intermetallic compounds or excess phases during the aging of martensite. At the same time, tempering can be performed both once and twice - in order to increase the impact strength.

An example of using the invention

In order to optimize the claimed steel composition and component ratio in accordance with the task, experimental melts were melted, the compositions of which are shown in Table 1. Steel was smelted in an electric arc furnace and subjected to vacuum-oxygen refining in a ladle. Ingots weighing 1.15 tons were cast, which were squeezed on blooming into a square of 100 mm, and then on a continuous mill "250" rolled into bars with a diameter of 19 mm. The rolling of the rods was completed at a temperature within the interval of 970-1050 ° C and one part of the rods without heat treatment after rolling was subjected to turning on the Kizerling line, followed by annealing at 740 ° C and tempering at 540 ° C with a holding time of 3 hours. The other part of the bars after rolling was subjected to normalization from 950 ° C, followed by tempering at 740 ° C and 540 ° C with a holding time of 3 hours each. In laboratory conditions, the bar heat treatment mode was tested after rolling - cooling to minus 70 ° with a holding time of 2 hours and tempering at 540 ° C. Samples according to GOST 7564-97 and GOST 7565-64 were cut out from rods after heat treatment to determine the mechanical tensile properties according to GOST 1497-84, impact strength according to GOST 9454-78. The resistance of steel to stress corrosion cracking was determined by the method of NACE TM 0177-96 (USA). Samples were placed in medium from a 5% aqueous solution of NaCl and a 0.5% solution of hydrogen sulfide and subjected to a tensile force of 80 kG / mm ² at a temperature of 20 ° C until failure. The test results are shown in table 2.

Compositions No. 2, 3, 4, 5, 7 satisfy the requirements of the claimed invention, namely KCU ₊₂₀ > 7 kgf · m / cm ² and yield strength at break 110-150 kgf / mm ² . The optimal composition is No. 5. In composition No. 1, Cr is introduced above the upper limit, due to which the ratio Cr _eq. / Ni _equiv. > 1.7 and a structure (Ф + М + А) was formed in steel with reduced strength and low impact strength. In composition No. 6, due to the high content of Cr and Cu at the upper limit, an (M + A) structure is formed with a large amount of austenite, low strength, and high viscosity. Composition No. 10 with a high Cr content has Ni at the lower limit, the ratio of Cr _equiv. / Ni _equiv. <1.7, a structure M + F + A arises with a sufficiently high strength and low viscosity. Composition No. 9 has Ni below the lower limit and a Cr _equiv ratio is obtained _. / Ni _equiv. > 1.7, steel has reduced strength and toughness characteristics. Composition No. 8 has Cr at the lower limit, Ni at the upper limit, but the Ti and Al content is higher than the upper limit, due to which Ni was bound into intermetallic compounds, steel has high strength but low viscosity. At the optimum composition No. 5 of steel, the effect of the temperature of rolling completion on the properties was evaluated. The results are shown in table 3.

Table 1 Chemical composition, wt.% structure FROM Mn Si Cr Ni Cu Mo Ti Al Co N Nb AT
by calculation Ca by calculation Xe for calculation S R Cr _eq. / Ni _equiv. one 0.02 0.5 0.4 17.5 7.0 0.3 1,0 0.2 0.5 0.01 0.05 0.2 0.005 0.02 0.02 0,03 0,03 2,56 2 0,03 0.6 0.5 12 8.0 1.8 0.8 0.2 0.2 2.0 0.1 0.3 0.005 0.02 0.02 0.02 0,03 1,5 3 0.02 0.5 0.4 14.5 5,0 4,5 0.05 0.01 0.01 0.01 0.1 0.3 0.005 0.02 0.02 0.02 0,03 1,59 four 0.02 0.6 0.4 10.0 7.0 2.5 1.8 0.6 0.1 1,0 0.05 0.35 0.005 0.02 0.02 0.005 0,03 1.39 5 0.02 0.4 0.4 11.0 11.0 0.05 2.0 0.4 0.01 3.0 0.05 0.3 0.005 0.02 0.02 0.015 0,025 1.16 6 0.02 0.6 0.3 16.5 5,0 5,0 0.05 0.01 0.01 1,0 0.1 0.2 0.005 0.02 0.02 0.02 0,03 1,57 7 0,03 0.6 0.6 13.0 7.0 2.0 0.5 0.01 0.01 0.01 0.1 0.2 0.005 0.02 0.02 0,03 0,025 1.37 8 0.02 0.6 0.4 8.0 8.0 1,5 2.5 0.9 0.9 1,0 0.15 0.3 0.005 0.02 0.02 0.02 0,03 1.51 9 0.02 0.6 0.5 12.0 3.8 0.5 0.1 0.1 0.1 0.01 0.1 0.2 0.005 0.02 0.02 0,03 0,03 2.42 10 0.02 0.6 0.4 16,2 4.0 2.2 0.05 0.01 0.01 0.01 0.05 0.3 0.005 0.02 0.02 0,03 0,03 2,4

table 2 Structure σ _in , kgf / mm ² σ _t , kgf / mm ² δ,% ψ,% KCU, kgf / mm ² Time to destruction, hours Estc one 120 105 9 35 6.5 1050 21.6 2 137 130 10 52 8.0 1210 16.24 3 125 115 12 65 12.0 1120 16,2 four 135 130 10 fifty 8.0 1320 16.74 5 155 150 10 fifty 7.0 1440 18,4 6 95 80 13 60 15.0 - 17.5 7 130 125 10 58 8.0 1120 16.25 8 170 165 5 25 3.0 1230 17.85 9 110 90 12 58 7.0 550 13.93 10 125 117 12 54 6.5 1130 17,165

Table 3 deformation end temperature, ° C σ _in , kgf / mm ² σ _t , kgf / mm ² δ,% ψ,% KCU, kgf / mm ² 1020 155 150 10 fifty 7.0 900 156 152 8.0 45 5.7 1100 145 140 eleven 52 6.2

Claims (5)

1. Stainless high-strength steel containing carbon, chromium, nickel, copper, titanium, aluminum, molybdenum, niobium, cobalt, nitrogen, calcium, boron, cerium, silicon, manganese, sulfur, phosphorus, iron and related impurities, characterized in that it contains these components in the following ratio, wt.%:
carbon (C) ≤0.03 chromium (Cr) 8.0 to less than 15.0 nickel (Ni) 4.0-12.0 copper (Cu) 0.3-5.0 titanium (Ti) 0.01-1.0 aluminum (Al) 0.01-0.6 molybdenum (Mo) 0.05-3.0 niobium (Nb) 0.05-0.4 cobalt (Co) 0.01-3.0 nitrogen (N) 0.005-0.15 calcium (Ca) ≤0.02 boron (B) ≤0.005 cerium (ce) ≤0.02 silicon (Si) ≤0.8 Manganese (Mn) ≤0.8 sulfur (S) ≤0.03 phosphorus (P) ≤0.03 iron and related impurities rest
while ferrite-forming and austenite-forming elements with their equivalence coefficients are related by the relation

. 2. Steel according to claim 1, characterized in that it is obtained by electric arc smelting with vacuum-oxygen refining in a ladle, followed by hot rolling and heat treatment. 3. Steel according to claim 2, characterized in that the temperature of the end of hot rolling is limited in the range from 970 to 1050 ° C. 4. Steel according to claim 2, characterized in that the heat treatment includes at least one mode in the temperature range from -70 to 950 ° C with a holding time of at least 2 hours 5. Steel according to any one of claims 1 to 4, characterized in that it has a tensile strength in the range from 1078-1470 MPa, impact strength of at least 68.6 J / cm ² and pitting corrosion resistance equivalent to> 15.