RU2445397C1 - High-strength non-magnetic corrosion-resistant cast steel, and item made from it - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains the following, wt %: carbon ≤ 0.06, silicium 0.1 - 1.0, chrome 19.0 - 23.0, manganese 14.0 -16.0, nickel 6.0 - 9.0, molybdenum 0.5 - 1.5, nitrogen 0.45 - 0.67, vanadium 0.01 - 0.50, niobium 0.01 - 0.30, yttrium 0.001 - 0.050, calcium 0.005 - 0.010, boron 0.001 - 0.01, and iron and inevitable impurities are the rest. Steel can also contain one or several elements of the group, wt %: tungsten 0.1 - 0.5, copper 0.01 - 2.0, cobalt 0.01 - 2.0, titanium 0.001 - 0.3, tantalum 0.001 - 0.3, zirconium 0.01 - 0.3, selenium 0.001 - 0.02. Steel composition meets the condition: 1.17*Cr_equiv-Ni_equiv<11.164, where Ni_equiv=[Ni] + 0.1[Mn] - 0.01[Mn]²+ 18[N] + 30[C] +0.5[Cu]+0.5[Co], Cr_equiv=[Cr] + 1.5[Mo] + 0.48[Si] + 2.3[V] + 1.75[Nb] + 1.5[Ti] +0.75[W]+1.75[Ta].

EFFECT: steel has high strength, ductility, corrosion resistance and weldability.

6 cl, 2 tbl

Description

The invention relates to the field of metallurgy, in particular to high-strength foundry non-magnetic corrosion-resistant steels, and can be used for the manufacture of cast products used in shipbuilding, mechanical engineering, including chemical, for the oil and gas industry.

Known cast austenitic corrosion-resistant welded steel 12X18H10TL (GOST 977-88, OST 108.961.04-80), containing, wt.%: Carbon ≤0.12, silicon 0.2-2.0, manganese 1.0-2.0 , chromium 17.0-20.0, nickel 8.0-11.0, Ti 5 * C - 0.70, sulfur ≤0.030, phosphorus ≤0.035. The disadvantage of this steel is its low strength (yield strength of not more than 200 MPa) and relatively low corrosion resistance (its equivalent to pitting corrosion resistance is ESP = 17-20).

Of austenitic corrosion-resistant steels used as cast material, CF3MN steel is known, which was developed on the basis of a deformable analogue - steel 316LN (ASTM standard A 351, A 743, A 744, UN No. UN92 - Unified numbering system - a unified system of chemical compositions cast corrosion-resistant steels (http://www.stainless-steel-world.net/pdf/11022.pdf). This steel contains, wt.%: carbon ≤0.030, nitrogen 0.10-0.20, silicon ≤1.50, manganese ≤ 1.50, chromium 17.0-21.0, nickel 9.0-13.0, molybdenum 2.00-3.00, sulfur ≤0.040, phosphorus ≤0.040. It has good corrosion resistance to pitting corrosion (ESP can reach 34.1) A significant drawback of this casting steel is its low yield strength (about 250 MPa) and the presence of up to 10–20% ferrite in the steel structure.

Closest to the proposed steel is cast high-strength non-magnetic steel, as well as products made from it, disclosed in Japanese Patent No. 2104633, published 04/17/1990. Steel contains in wt.%: <= 1.5 carbon, <= 3.0 silicon, <= 15 chromium, 7-40 manganese, <= 10 nickel, <= 3.0 molybdenum, <= 0.4 nitrogen, <= 2.0 niobium, 0.0005-0.050 calcium, <= 0.5 boron and <= 0.5 rare earth element. Despite the fact that steel has good strength characteristics, their values are insufficient for the manufacture of highly loaded parts and structures from it. In addition, it has a low corrosion resistance, which limits its use for the manufacture of products operating in aggressive environments.

The problem to which the invention is directed, is the creation of cast non-magnetic steel with high strength, toughness and corrosion resistance.

The technical result of the invention is to increase the strength, corrosion resistance and toughness of cast non-magnetic steel.

The technical result is achieved in that a high-strength casting non-magnetic corrosion-resistant steel containing carbon, silicon, chromium, manganese, nickel, molybdenum, nitrogen, vanadium, niobium, rare earth element, calcium, boron, iron and inevitable impurities, according to the invention as a rare earth element contains yttrium in the following ratio of components, wt.%:

carbon ≤0.06 silicon 0.1 ÷ 1.0 chromium 19.0 ÷ 23.0 manganese 14.0 ÷ 16.0 nickel 6.0 ÷ 9.0 molybdenum 0.5 ÷ 1.5 nitrogen 0.45 ÷ 0.67 vanadium 0.01 ÷ 0.50 niobium 0.01 ÷ 0.30 yttrium 0.001 ÷ 0.050 calcium 0.005 ÷ 0.010 boron 0.001 ÷ 0.01 iron and inevitable impurities rest.

The steel may also contain at least one element selected from the group, wt.%: 0.01-0.5 tungsten; 0.01-2.0 copper; 0.01-2.0 cobalt; 0.001-0.3 titanium, 0.001-0.3 tantalum, 0.01-0.3 zirconium, 0.001-0.02 selenium. Moreover, the total content of niobium, vanadium, titanium, tantalum and zirconium should not exceed 0.35 wt.% And the following condition is fulfilled:

1.17 * Cr ^, _eq- Ni ^, _eq <11.164,

where Ni ^, _equiv - equivalent, taking into account the austenite-forming action of alloying elements, calculated by the formula

Ni ^, _equiv = [Ni] +0.1 [Mn] -0.01 [Mn] ² +18 [N] +30 [C] +0.5 [Cu] +0.5 [Co];

Cr ^, _equiv - equivalent, taking into account the ferrite-forming action of alloying elements, calculated by the formula

Cr ^, _equiv = [Cr] +1.5 [Mo] +0.48 [Si] +2.3 [V] +1.75 [Nb] +1.5 [Ti] +0.75 [W] + 1.75 [Ta],

[N], [C], [Si], [Mn], [Ni], [Cr], [Mo], [V], [Nb] [Cu], [Co], [Ti], [W] , [Ta] is the concentration in the steel of nitrogen, carbon, silicon, manganese, nickel, chromium, molybdenum, vanadium, niobium, copper, cobalt, titanium, tungsten and tantalum (wt.%). Steel is characterized by a high-strength austenitic structure in the cast state, as well as after homogenizing heat treatment and after aging, and can be designed for welding.

The technical result is also achieved in a molded product made of the declared steel.

The invention consists in the following.

To improve the casting properties in the declared steel increased nitrogen content. To obtain high strength and satisfactory viscosity of the base metal, as well as high-quality welded joints, the chemical composition of the steel should provide:

high solubility of nitrogen in the liquid metal and crystallization without the formation of δ-ferrite, which determines the high nitrogen content in γ solid solution; stabilization of austenite of the weld and base metal with respect to γ → α, γ → σ, γ → ε transformations; the formation of a structure with a small amount of nitrides of the MeN type (for grinding austenitic grains) without carbides of the Me ₂₃ C _{6 type} in the base metal and δ-ferrite in the weld.

The content in the steel of a minimum amount of nitrogen - 0.45% in combination with carbon in an amount of <0.06 wt.% Is sufficient to ensure the high strength of the cast metal, including and in a welded joint. Carbon and nickel reduce the solubility of nitrogen in steel; therefore, with a content of more than 0.06% C and 9.0% Mi, more than 0.4% nitrogen cannot be dissolved in this steel. The introduction of chromium in steel in an amount of 19-23% and molybdenum in an amount of 0.5-1.5% is necessary to ensure the required level of corrosion resistance and solubility of nitrogen within the specified limits of the inventive steel. With a chromium content of more than 23% and nickel less than 6%, the steel will have a reduced ductility due to the formation of ferrite and σ phase. A manganese content of 14–16% ensures the stability of austenite with respect to the γ → α transformation and the given solubility of nitrogen in steel. In addition, manganese increases the fluidity of steel. The introduction of vanadium in the amount of 0.01 ÷ 0.50% provides a fine-grained structure and an increase in strength due to the formation of finely dispersed vanadium nitrides. When the content of V <0.01%, the effect of hardening from its introduction is negligible. The introduction of vanadium in an amount of more than 0.50% leads to a decrease in strength due to the depletion of the solid solution with nitrogen as a result of the formation of vanadium nitrides, which dissolve in austenite at temperatures above 1150 ° C. When the molybdenum content is more than 1.5%, a ferromagnetic phase (δ-ferrite) can form in the metal. The content of silicon steel in an amount of 0.3% is common, obtained by the smelting of corrosion-resistant steels deoxidized by silicon. Obtaining a lower silicon content in steel will require special smelting technology. The introduction of silicon in an amount of 0.1-1.0% is due to the fact that silicon increases the fluidity of steel, however, exceeding this concentration> 1.0% is impractical, because it reduces the solubility of nitrogen in steel and the ductility characteristics of solid metal. Additives of calcium in an amount of 0.005-0.01% and yttrium in an amount of 0.001-0.05%, improving the morphology of non-metallic inclusions, increase the manufacturability of steel, especially machinability by cutting. If the content of calcium and yttrium in the metal is less than 0.005 and 0.001%, respectively, a significant effect from their introduction is not observed. With an increase in calcium content of more than 0.01% and yttrium more than 0.05%, further improvement in properties is not observed. The introduction of boron in an amount up to 0.010% leads to grain refinement, and with an increase in its content above 0.01%, it can lead to a deterioration in plastic properties and manufacturability in the process of plastic deformation. Fulfillment of the condition

1.17 * Cr ^, _eq- Ni ^, _eq <11.164,

where Ni ^, _equiv = [Ni] +0.1 [Mn] -0.01 [Mn] ² +18 [N] +30 [C] +0.5 [Cu] +0.5 [Co];

Cr ^, _equiv = [Cr] +1.5 [Mo] +0.48 [Si] +2.3 [V] +1.75 [Nb] +1.5 [Ti] +0.75 [W] + 1.75 [Ta]

provides a non-magnetic structure (µ <1.01 G / e), if 1.17 * Cr ^, _eq- Ni ^, _eq ≥ 11.164, then δ-ferrite appears in the steel.

Additional alloying with tungsten, copper, cobalt, titanium, tantalum, zirconium leads to an even greater increase in the strength of steel. So tungsten and cobalt helps increase the strength of the γ-solid solution. Alloying with copper allows hardening of steel during aging, due to precipitation of the nanoscale copper-containing phase. The limits of doping with these elements are due to the fact that when the content of tungsten, cobalt and copper is less than 0.01% there will be no effect of hardening of the solid solution. A tungsten content greater than 0.5% is not practical due to its action as a ferrite-forming element. A cobalt content higher than 2% is impractical for economic reasons. The introduction of copper in amounts above 2% can lead to cracking of the steel and also increases the cost of steel. Ti, Ta, Zr are introduced into steel to inhibit grain growth by fixing grain boundaries with particles of excess phases based on them. When the content of Ti, Ta, Zr is less than 0.001, the effect of restraining grain growth is not achieved, and when the content of these elements is more than 0.3%, the toughness and corrosion resistance are reduced. With a total content of vanadium, niobium, titanium, tantalum, zirconium above 0.35 wt.%, The steel loses its ductility.

Steel was smelted in an open induction furnace with a capacity of 150 kg. Castings were made by pouring metal into sandy forms based on HTS. The temperature-time parameters of the solidification of the castings were controlled according to the readings of thermocouples. The chemical composition of steel castings of the recommended compositions is given in table 1.

Table 1 The content of alloying elements in cast steels of the recommended composition No. pl. FROM N Cr Mn Ni Mo V Si Nb Y AT Ca Ni ^, _eq Cr ^' _eq [C] / [N] 1.17 * Cr ^, _eq- Ni ^, _eq one 0,031 0.45 19.1 14.1 7.1 1,5 0.06 0.63 0.05 0.001 0.001 0.005 14.76 21.88 0.08 10.94 2 0.04 0,501 21.6 15.0 8.7 0.9 0.26 0.94 0.12 0,025 0.006 0.007 18.18 24.21 0.08 10.15 3 0.058 0.670 23.0 16,0 6.2 0.5 0.49 0.31 0.29 0,050 0.01 0.010 19.08 25.53 0.09 10.79

As can be seen from table 1, for steels of the recommended composition 1–3, the condition 1.17 × Cr ^, _eq- Ni ^, _eq <1.164 is met to obtain non-ferromagnetic steel. Magnetic testing confirmed that the steel of these castings is austenitic in a cast and heat-treated state.

The surface of the castings has no signs of burning (metallized, chemical, etc.). Steel does not show a tendency to crack formation. Captures and twists on the surface of the castings were not observed.

Templates were cut from castings to study the structure. It was found that cast metal has a dense structure, in which there are no pores, cracks and other defects characteristic of cast products.

Also, metal was cut from the castings for the manufacture of samples, and mechanical properties were studied. Tensile tests were performed according to GOST 1497 and impact bending according to GOST 9454. According to the test results (table 2), it is seen that cast steel 1-3 of castings has high strength, ductility and toughness, superior to similar steel in strength of 1, 5-2 times, and the strength of steel is higher, the higher the concentration of nitrogen and carbon penetration elements in it. An assessment of pitting corrosion ESP = PREN =% Cr + 3.3 (% Mo) +16 (% N) showed that the steel of the recommended compositions is superior to steel-analogue. Cast products made from the claimed steel composition also have high strength, ductility and corrosion resistance.

table 2 Mechanical properties of austenitic steel castings and ESPs of recommended compositions after heat treatment (quenching from 1050-1100 ° С) No. pl. % FROM % N σ _0.2 (MPa) σ _B (MPa) δ (%) ψ (%) KCU, MJ / m ² ESP one 0,031 0.45 396 679 38 53 2.15 30,45394 2 0.04 0,501 430 750 36 58 2.73 32.59217 3 0.058 0.672 510 801 35 51 2.44 35.4063

In order to check the weldability of steel of the recommended compositions, plates were cut from the metal of the castings, which were welded by argon-arc and submerged arc welding, using a steel rod of the same composition as a welding material. The study of the structure of welded joints showed the absence of pores and cracks in them. In tensile and impact tests (in the absence of heat treatment of welded joints), it was found that the weld metal is not inferior to the base metal in strength and ductility, and its impact strength is at least 0.75% of the impact strength of the base metal.

Claims (6)

1. High-strength casting non-magnetic corrosion-resistant steel containing carbon, silicon, chromium, manganese, nickel, molybdenum, nitrogen, vanadium, niobium, rare earth element, calcium, boron, iron and inevitable impurities, characterized in that as a rare-earth element it contains yttrium in the following ratio of components, wt.%:
carbon ≤0.06 silicon 0.1 ÷ 1.0 chromium 19.0 ÷ 23.0 manganese 14.0 ÷ 16.0 nickel 6.0 ÷ 9.0 molybdenum 0.5 ÷ 1.5 nitrogen 0.45 ÷ 0.67 vanadium 0.01 ÷ 0.50 niobium 0.01 ÷ 0.30 yttrium 0.001 ÷ 0.050 calcium 0.005 ÷ 0.010 boron 0.001 ÷ 0.01 iron and inevitable impurities rest 2. Steel according to claim 1, characterized in that it further comprises at least one element selected from the group, wt.%: Tungsten 0.01-0.5, copper 0.01-2.0, cobalt 0.01-2.0, titanium 0.001-0.3, tantalum 0.001-0.3, zirconium 0.01-0.3, selenium 0.001-0.02, and the total content of niobium, vanadium, titanium, tantalum in steel and zirconium does not exceed 0.35 wt.% and the following condition is met:
1.17 · Cr ^, _eq- Ni ^, _eq <11.164,
where Ni ^, _equiv - equivalent, taking into account the austenitic effect in steel nickel [Ni], manganese [Mn], nitrogen [N], carbon [C], copper [Cu] and cobalt [Co], calculated by the formula, wt.%:
Ni ^, _equiv = [Ni] +0.1 [Mn] -0.01 [Mn] ² +18 [N] +30 [C] +0.5 [Cu] +0.5 [Co],
Cr ^, _eq - equivalent taking into account the ferrite-forming action in steel of chromium [Cr], molybdenum [Mo], silicon [Si], vanadium [V], niobium [Nb], titanium [Ti], tungsten [W] and tantalum [Ta] calculated by the formula, wt.%:
Cr ^, _equiv = [Cr] +1.5 [Mo] +0.48 [Si] +2.3 [V] +1.75 [Nb] +1.5 [Ti] +0.75 [W] + 1.75 [Ta]. 3. Steel according to claim 1 or 2, characterized in that it has an austenitic structure in a molten state. 4. Steel according to claim 1 or 2, characterized in that after homogenizing heat treatment, as well as homogenizing heat treatment and aging, it has an austenitic structure. 5. Steel according to claim 1 or 2, characterized in that it is a welded steel. 6. The product is made of a high-strength foundry non-magnetic corrosion-resistant steel, characterized in that it is made of steel according to any one of claims 1 to 5.