RU2806682C1 - High strength corrosion resistant nitrogen containing martensitic-austenitic-ferritic steel - Google Patents

Abstract

FIELD: steel metallurgy.

SUBSTANCE: alloyed corrosion-resistant high-strength martensitic-austenitic-ferritic steels used for the manufacture of highly loaded parts in mechanical engineering. Steel contains components in the following ratio, wt.%: carbon 0.02-0.05, nitrogen 0.11-0.20, chromium 17.5-20.0, manganese 1.0-2.0, nickel 2, 0-3.0, vanadium 0.07-0.13, silicon 0.2-0.4, cerium 0.005-0.03, lanthanum 0.005-0.03, barium 0.005-0.02, iron and other impurities. The content of components in steel satisfies the following conditions: Ni_eqv=%Ni+0.1%Mn-0.01%Mn²+30%C+18%N=4.67÷8.36 and Cr_eqv=%Cr+0.48%Si+2.3%V=17.76÷20.49.

EFFECT: increase in ductility and impact strength is ensured while maintaining high strength, which ensures increased operational reliability and an increase in the service life of structures made from this steel during their operation.

4 cl, 2 tbl

Description

The invention relates to the field of steel metallurgy, in particular to high-strength corrosion-resistant steels of the martensitic-austenitic-ferritic class, strengthened with nitrogen, which, along with high strength, have increased values of ductility and impact toughness, including at low (down to - 70 ° C) temperatures.

The steel is intended for the manufacture of highly loaded machine parts - blades, disks, shafts, compressors of oil and gas production equipment operating at low temperatures (up to - 70°C).

High-strength, corrosion-resistant, low-nickel martensitic-ferritic steel 14Х17Н2 (EI 268, GOST 5632 - 72) is known, with the following chemical composition, wt.%:

carbon 0.11-0.17 chromium 16.0-18.0 nickel 1.5-2.5 manganese ≤0.8 silicon ≤0.8 iron and impurities rest

The main disadvantages of this steel are low ductility (δ = 10%) and a tendency to temper brittleness. This steel, after quenching and tempering at 550°C, has low impact strength (KCU = 0.1 - 0.3 MJ/m ² ) [Liu Ning, Deng Zhonggand, Huang Menggen. Effect of heat treatment on microstructure and mechanical properties of martensitic-ferritic stainless steel containing 17% Cr and 2% Ni. Materials Science and Technology. November. V.7. p.1057. 1991]. AISI 431 steel, which is similar to 14Х17Н2 steel in chemical composition and purpose, has the same disadvantages. AISI 431 steel containing 0.17% C, 16% Cr, 1% Mn and 2% Ni, after quenching and tempering at 670°C + 600°C, has low ductility (δ = 6.8 - 13.5%) [A. Rajasekhar, G. Madhusudhan Reddy, T. Mohandas, VSR Murti. Materials and Design. 2009. V.30. p.1612 - 1624]. The low ductility of this steel with a high content (0.17%) of carbon is associated with the presence in its structure after the specified heat treatment of large particles of carbide of the Cr ₂₃ C ₆ type, which negatively affect the ductility, toughness, corrosion resistance and weldability of corrosion-resistant steels.

The closest to the declared chemical composition is steel (patent No. JP 2003089851, Japan), containing, wt.%:

carbon 0.04-0.15 nitrogen ≤0.1 chromium 10.0-20.0 manganese ≤2 nickel 0.5-4.0 vanadium 0-0.3 silicon ≤2 copper ≤2 molybdenum 0-2.0 niobium 0-0.5 aluminum 0-0.05 titanium 0-0.5 calcium 0-0.1 yttrium 0-0.2 boron 0-0.015

This steel has a martensitic-ferritic, austenite-free structure. A significant disadvantage of this steel is the increased carbon content (0.04-0.15%) and low nitrogen content (≤0.1%), at which during heat treatment carbides of the Cr ₂₃ C ₆ type will be formed, reducing ductility and impact strength steel and corrosion resistance. Another significant disadvantage of this steel is its low structural stability, which does not ensure the stability of mechanical and corrosion properties (large range of values) due to too wide alloying limits, which makes it impossible to use it for the production of highly loaded parts and structures. Also, a disadvantage of this steel is the presence in its composition of expensive alloying elements, such as Mo, Y, Cu, Ti, N, which lead to an increase in the cost of the final semi-finished product or product.

The technical result of the proposed invention is the creation of economically alloyed steel, which does not contain expensive alloying elements (Mo, Y, Cu, Ti, Nb), with a stable martensitic-austenitic-ferritic fine-grained structure without carbides of the Cr ₂₃ C ₆ type, providing increased ductility, impact strength and corrosion resistance while maintaining high strength.

The technical result is achieved due to the additional introduction into high-strength, corrosion-resistant nitrogen-containing martensitic-austenitic-ferritic steel containing carbon, nitrogen, chromium, nickel, manganese, silicon, vanadium, iron and inevitable impurities of cerium, lanthanum and barium in the following ratio of components, wt.% :

carbon 0.02-0.05 nitrogen 0.11-0.20 chromium 17.5-20.0 manganese 1.0-2.0 nickel 2.0-3.0 vanadium 0.07-0.13 silicon 0.2-0.4 cerium 0.005-0.03 lanthanum 0.005-0.03 barium 0.005-0.02 iron and impurities rest

In this case, the ratio of nitrogen to carbon content of high-strength corrosion-resistant nitrogen-containing martensitic-austenitic-ferritic steel is 0.18-0.25, and the ratio of austenite- and ferrite-forming elements that determine the phase composition in steel should be determined by the following equalities:

(1)

(2)

,

at the same time, high-strength corrosion-resistant nitrogen-containing martensitic-ferritic - austenitic steel after hardening from a temperature of 800-850 ° C has a stable martensitic - austenitic - ferritic fine-grained (with a ferrite grain size of 4 microns) structure with a specified content of martensite 74%, ferrite 18% and austenite 8 %, not containing carbides such as Cr ₂₃ C ₆ .

The elements vanadium, carbon and nitrogen form dispersed particles of vanadium nitrides (carbonitrides) of type V (C, N) in steel, with a ratio of nitrogen to carbon content of 0.18-0.25, which inhibit grain growth in steel when heated for quenching, t .e. contribute to the formation of the fine-grained steel structure necessary to obtain high strength.

Equations (1-2) regulate the amount of martensite, ferrite and austenite in steel by taking into account the ferrite- and austenite-forming ability of the elements included in the steel (using the coefficients from the equations to estimate the equivalents of ferrite and austenite formation). The ratio of components within the framework of equations 1 and 2 allows us to obtain an optimal structure with a given amount of martensite (74%), ferrite (18%) and austenite (8%).

Boundary conditions for the concentrations of individual elements included in the steel composition are determined by the following factors. Steel with a chromium content of less than 17.5% and nickel content of less than 2% (pl. 5, tables 1, 2) does not achieve sufficient ductility for practical use (δ > 10%), due to the formation of a structure containing a small amount of austenite (< 3%).

When the concentration of chromium is more than 20%, nickel is more than 3% and nitrogen is more than 0.20% (pl. 6. Tables 1, 2), it is impossible to obtain high-quality large pore-free ingots due to the low solubility of nitrogen in liquid steel at this chromium content and nickel A nitrogen content of less than 0.11% (heat 5, tables 1, 2) does not provide sufficient strength, and more than 0.20% (heat 6, tables 1, 2) does not provide steel with ductility. The introduction of more than 0.05% carbon into the claimed steel (heat 6, tables 1, 2) leads to a decrease in ductility as a result of the predominant precipitation of large particles of chromium carbide of the Cr ₂₃ C ₆ type along the grain boundaries.

To obtain steel with a carbon content of less than 0.02%, a special smelting technology is required. Additions of vanadium in an amount of 0.07-0.13% ensure a fine-grained structure. Vanadium additives in quantities less than 0.07% are ineffective, and at their concentration above 0.13%, low values of steel ductility characteristics are possible due to the high content of vanadium carbonitrides.

Additional alloying with barium in an amount of 0.005-0.02% makes it possible to change the shape of sulfides to globular and thereby improve the deformability of ingots and increase ductility. When barium is introduced in an amount of less than 0.005%, no significant change in the form of sulfides is observed. The introduction of barium in an amount of more than 0.02% leads to the enlargement of sulfides and a decrease in impact strength.

The presence of cerium and lanthanum in an amount of 0.005-0.030%, which are active deoxidizers, leads to a decrease in the number of non-metallic inclusions in the metal, making them round, and also contributes to the grinding of structural components, which also increases ductility and toughness. When introducing cerium and lanthanum in an amount of less than 0.005%, a significant reduction in the amount of non-metallic inclusions is not observed. When introducing cerium and lanthanum in quantities of more than 0.03%, it leads to the enlargement of non-metallic inclusions, which, being stress concentrators, lead to a decrease in the ductility and toughness of steel.

The chemical composition and mechanical properties of the studied melts of the proposed and known steels are given in Tables 1 and 2.

The steel was smelted in an open induction furnace. Heat treatment was carried out according to regimes consisting of hardening from 800 - 850°C with cooling in water.

The proposed steel (pl. 2, 3, 4) after hardening at 800 and 850°C exceeds the prototype steel (14Х17Н2) in terms of ductility and impact toughness. Table 2 shows the mechanical properties of the prototype steel 14X17H2 and the proposed steel. After hardening from 800-850°C and cooling in water, the inventive steel has increased values of ductility (δ = 16-18%, ψ = 43-46%) and impact strength (KCU ^+20°C = 1.28-1, 67 MJ/m ² , KCU ^-70°C = 0.67-0.75 MJ/m ² ) with approximately the same tensile strength (σ _B = 1065-1068 MPa) and yield strength (σ _0.2 ) as for steel 14Х17Н2 =851-872 MPa). This combination of mechanical properties of the proposed steel is achieved as a result of the formation of a fine-grained structure with a grain size of 4 microns, with a given content of martensite 74%, ferrite 18% and austenite 8%, without Cr ₂₃ C ₆ carbides, which reduce ductility and corrosion resistance.

The proposed steel can thus be used for the manufacture of highly loaded machine parts - blades, disks, shafts, compressors of oil and gas production equipment operating at low temperatures (up to - 70°C).

Claims (9)

1. High-strength corrosion-resistant nitrogen-containing martensitic-austenitic-ferritic steel containing carbon, nitrogen, chromium, nickel, manganese, silicon, vanadium, iron and inevitable impurities, characterized in that it additionally contains cerium, lanthanum and barium in the following ratio of components, wt .%: carbon 0.02-0.05 nitrogen 0.11-0.20 chromium 17.5-20.0 manganese 1.0-2.0 nickel 2.0-3.0 vanadium 0.07-0.13 silicon 0.2-0.4 cerium 0.005-0.03 lanthanum 0.005-0.03 barium 0.005-0.02 iron and impurities rest 2. Steel according to claim 1, characterized in that the ratio of nitrogen to carbon content is 0.18-0.25. 3. Steel according to claim 1, characterized in that the ratio of austenite- and ferrite-forming elements that determine the phase composition in steel is determined by the following equalities: (1) (2) . 4. Steel according to claim 1, characterized in that after quenching from a temperature of 800-850°C it has a stable martensitic-ferritic-austenitic fine-grained structure containing 74% martensite, 18% ferrite and 8% austenite without carbides such as Cr ₂₃ C ₆ , while the ferrite grain size is 4 µm.