RU2447185C1 - High-strength nonmagnetic rustproof casting steel and method of its thermal treatment - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed steel contains the following components, in wt %: carbon - 0.03-0.06, silicon - 0.10-0.40, manganese - 14.0-16.0, chromium - 19.00-20.5, nickel - 8.25-9.0, molybdenum - 0.8-1.25, vanadium - 0.08-0.15, niobium - 0.02-0.12, nitrogen - 0.57-0.65, titanium - 0.004-0.03, cerium - 0.005-0.02, calcium - 0.005-0.02, aluminium - 0.005-0.02, iron and impurities making the rest. Steel is subjected to homogenising treatment with heating and cooling. Then, stepwise heating is carried out to 850°C, 950°C and 1100-1150°C, while cooling is preformed in water.

EFFECT: high mechanical properties, fine grain as-cast.

3 cl, 3 tbl

Description

The invention relates to the field of metallurgy, in particular to high-strength corrosion-resistant casting steels, in particular to the creation of steels that can be used for casting a number of non-magnetic highly loaded parts operating under conditions of intense corrosion in power engineering and other fields.

The invention can be most effectively used in the manufacture of highly efficient equipment for special shipbuilding, drilling equipment and mechanical engineering.

Known for these purposes is corrosion-resistant non-magnetic austenitic steel of the POLARIT 774 class (Germany DIN 1.4539), it has the following chemical composition (wt.%):

Carbon ≤0.02 Silicon ≤0.7 Manganese ≤2.0 Sulfur ≤0.01 Phosphorus ≤0.03 Chromium 19.0-21.0 Nickel 24.0-26.0 Molybdenum 4.0-5.0 Nitrogen ≤0.15 Copper 1.20-2.0 Iron - the rest

The disadvantage of this steel with stable austenite is its low strength and high content of expensive nickel and molybdenum.

Known for these purposes is corrosion-resistant non-magnetic austenitic steel of the following composition (wt.%):

Carbon 0.04-0.09 Silicon 0.10-0.60 Chromium 19.0-21.0 Manganese 5.0-12.0 Nickel 4,5-9,0 Molybdenum 0.5-1.5 Vanadium 0.10-0.55 Niobium 0.03-0.30 Calcium 0.005-0.01 Nitrogen 0.40-0.70 Iron and impurities - the rest

(see Patent RU, 2205889, C1, Cl. C22C 38/58, 10.06.2003)

The disadvantage of this steel is a large interval in the content of the main alloying elements, which leads to a spread of data on the mechanical properties and structure. With the content of austenite-forming elements at the lower level and ferrite-forming elements at the upper level, δ-ferrite appears in the steel structure, which will not allow using this steel as non-magnetic.

Closest to the proposed steel in technical essence and the achieved result is steel of the following composition (wt.%):

Carbon 0.04-0.09 Silicon 0.10-0.60 Manganese 14.0-16.0 Chromium 21.0-23.0 Nickel 7.0-9.0 Molybdenum 1.0-2.0 Nitrogen 0.45-0.55 Vanadium 0.10-0.30 Cerium 0.001-0.030 Calcium 0.005-0.010 Boron 0.001-0.010 Iron and impurities - the rest

(see Patent RU, 2303648, C1, C22C 38/58, 07.27.2007)

Steel has worked well in the forged version. However, when using this steel in the cast version, intensive grain growth is observed, which leads to a decrease in mechanical properties. In addition, when the content of carbon, nitrogen and manganese is at the lower level, and silicon, chromium, molybdenum and vanadium are at the upper level, δ-ferrite may appear in the structure of large castings, which leads to a violation of the non-magnetic nature of steel.

EFFECT: obtaining a casting high-strength corrosion-resistant and high-viscosity non-magnetic steel with fine grain. This result is achieved by the fact that the proposed steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, nitrogen, cerium, calcium and iron, additionally contains niobium, titanium and aluminum, in the following ratio of components (wt.%):

Carbon 0.03-0.06 Silicon 0.10-0.40 Manganese 14.0-16.0 Chromium 19.00-20.5 Nickel 8.25-9.0 Molybdenum 0.8-1.25 Vanadium 0.08-0.15 Niobium 0.02-0.12 Nitrogen 0.57-0.65 Titanium 0.004-0.03 Cerium 0.005-0.02 Calcium 0.005-0.02 Aluminum 0.005-0.02 Iron and impurities - the rest

The ratio of the total content of vanadium, niobium, titanium and aluminum to the total content of carbon and nitrogen is 0.15-0.50, and to reduce dendritic segregation, homogenizing heat treatment was carried out, which consisted in heating to a temperature of 850 ° C, then heating to a temperature of 950 ° C, then heating to 1100-1150 ° C, and cooling to water.

The introduction of aluminum in the composition of steel in 0.005-0.02 wt.% In combination with chemically active elements calcium and cerium favorably changes the shape of non-metallic inclusions, reduces the oxygen and sulfur content in steel, reduces the amount of sulfide inclusions, cleans and strengthens grain boundaries and grinds the structure cast steel, which leads to increased strength, ductility and toughness. Calcium and cerium also have a favorable effect on the character of nitride inclusions and promote the transition of film inclusions of aluminum nitrides to globular complexes of oxysulfonitride formations. The combined effect of aluminum, calcium and cerium opens up additional possibilities in controlling the structure and properties of cast steel.

When the content of A1 is below the lower limit, its effect on the properties of steel is ineffective, and when it is above the upper limit, it causes excessive enrichment of grain boundaries with non-metallic inclusions, which negatively affects the properties of steel. In addition, with an excess Al content, the spillability of steel decreases sharply.

Microalloying cast steel with a high nitrogen content at the same time niobium (0.02-0.12 wt.%), Vanadium (0.08-0.15 wt.%) And titanium (0.004-0.03 wt.%) Increases the strength, ductility and toughness of heat-treated steel by grinding the actual grain, reducing the carbon content in martensite and increasing the forces of interatomic bonds and the value of tear resistance. After the optimal heat treatment of the steels, their hardening occurs, while maintaining high toughness due to the compensating effect of grain refinement. The carbides and nitrides of vanadium, niobium and titanium have similar crystal lattice parameters and have unlimited mutual solubility and form carbonitrides. Dissolution upon heating of niobium carbonitrides occurs at a higher temperature than vanadium compounds. Complete dissolution of vanadium carbonitrides ends at 800-900 ° C, and niobium carbonitrides at a temperature of about 1100 ° C. Aluminum, whose nitride dissolves in austenite at higher temperatures, also contributes to grain refinement and prevents its growth when heated.

An additional introduction of niobium 0.02-0.12 wt.% Promotes the binding of carbon to carbides and carbonitrides, which prevents the formation of chromium carbides at the grain boundaries. In addition, dissolution during heating of niobium carbonitrides occurs at a higher temperature than the formation of vanadium compounds at a temperature of about 1100 ° C, which contributes to the grinding of grain and prevents its growth during heating.

When the niobium content is below the lower limit, its effect on the grain size, and accordingly, on the strength and ductility is ineffective, and when the niobium content is above the upper limit, the amount of large carbides and carbonitrides increases, which will lead to a decrease in ductility.

An additional introduction of titanium of 0.004-0.03 wt.% Shifts the beginning of the formation of aluminum nitrides to a lower temperature region, which helps to prevent the release of film aluminum nitrides. The titanium carbonitride formed during the introduction of titanium into steel dissolves in austenite at a higher temperature - more than 1200 ° C, which contributes to an increase in strength and ductility due to titanium carbonitrides that impede grain growth during heating. Dispersed carbides and carbonitrides have a barrier effect on the migrating grain boundary. Titanium carbonitrides are more rounded and smaller in comparison with titanium nitrides. Titanium carbonitrides are distributed relatively evenly in the cast metal; some of these inclusions tend to concentrate in the branches of the dendrites and in the interdendritic space.

When the titanium content is below the lower limit, its effect on the grain size, and accordingly, on the strength and ductility is ineffective, and when the titanium content is above the upper limit, the number of large acute-angled titanium nitrides increases, which will lead to a decrease in ductility.

The proposed steel differs from the known lower carbon content of 0.03-0.06 wt.%, Against 0.04-0.09 wt.%, Which is optimal to ensure high processability and contributes to high strength, corrosion resistance and higher values ductility and toughness.

When the carbon content is below the lower limit, its effect on technological and service properties is ineffective, and when the carbon content is above the upper limit, the coalescence of carbides and depletion of the solid solution are accelerated, which reduces ductility and corrosion resistance.

The proposed steel differs from the known lower chromium content of 19.0-20.5 wt.%, Against 21.0-23.0 wt.%, Which is optimal for ensuring the stability of austenite and high corrosion resistance.

When the chromium content is below the lower limit, the solubility of nitrogen in the melt decreases, which reduces the strength of the steel, and when the chromium content is above the upper limit, some δ-ferrite may form and the non-magnetic nature of the steel is broken.

The proposed steel differs from the known lower molybdenum content of 0.8-1.25 wt.%, Against 1.0-2.0 wt.%, Which is optimal to ensure the stability of austenite and high corrosion resistance.

When the molybdenum content is below the lower limit, the corrosion resistance of the steel decreases, and when the molybdenum content is above the upper limit, especially when the content of ferrite-forming elements is at the upper level, the formation of the ferrite phase is possible, which will lead to a change in the non-magnetic nature of steel.

The proposed steel differs from the known high nitrogen content of 0.57-0.65 wt.%, Against 0.50-0.55 wt.%, Which is optimal to ensure the stability of austenite and high strength and corrosion resistance.

When the nitrogen content is below the lower limit, the stability of austenite of steel decreases, especially when the content of ferrite-forming elements is at the upper level, the formation of a ferrite phase is possible, which will lead to a change in the non-magnetism of steel, and when the nitrogen content is above the upper limit, the amount of large carbonitrides and nitrides increases, which leads to plasticity.

For an effective impact on the grain size of cast steel it is necessary to fulfill the condition: the ratio of the total content of vanadium, niobium, titanium and aluminum to nitrogen and carbon is 0.15-0.50. A coefficient value of less than 0.15 is undesirable, since the amount of carbides and carbonitrides formed is not enough to inhibit grain growth in cast steel, and a ratio of these elements of more than 0.50 leads to steel embrittlement due to the formation of large carbides and carbonitrides that form in the melt and do not dissolve during heat treatment.

The proposed steel differs from the known heat treatment mode, which consists in conducting a homogenizing heat treatment, including heating to a temperature of 850 ° C, then heating to a temperature of 950 ° C, then heating to 1100-1150 ° C, cooling to water. Quenching in water from temperatures of 1050-1100 ° C to provide the necessary strength characteristics as necessary, since changing the temperature of heating for hardening, we can increase the strength characteristics at a temperature of 1050 ° C and reduce at a temperature of 1100 ° C due to different alloying of the solid solution.

The temperatures of the heating stages are selected based on the dissolution temperatures of vanadium, niobium and titanium carbides and carbonitrides, which provides greater uniformity of the solid solution, compared with the mode for the prototype steel. For prototype steel, the heat treatment mode used is ordinary heating to a temperature of 1100 ° C followed by quenching in water, to a much lesser extent eliminates dendritic segregation (see Foundry, 2009, No. 6, pp. 23-28) compared to the proposed heat treatment mode.

Table 1 shows the chemical composition of the proposed steel 3 swimming trunks (1, 2, 3), as well as the composition of the steel of the prototype (4).

Smelting was carried out in a 150 kg induction furnace with metal casting on cast billets. To reduce dendritic segregation, a homogenizing heat treatment was carried out, which consisted in heating to a temperature of 850 ° C, holding for 2.5 hours, then heating to a temperature of 950 ° C, holding for 2.5 hours, then heating to 1100-1150 ° C, holding for 3 hours, chilled into water. Quenching in water at temperatures of 1050 ° C, holding for 3 hours.

Table 2 shows the mechanical properties obtained after optimal heat treatment.

Tensile tests were carried out on cylindrical samples of five times the length with a diameter of the calculated part of 6 mm in accordance with GOST 1497-84. Determination of impact strength at normal temperature was carried out on samples of type 11 according to GOST 9454-78. The phase composition of the metal was determined on a DRON-4 X-ray diffractometer.

As can be seen from table 2, the proposed steel has a significant advantage in terms of strength, ductility and toughness compared with steel prototype. The proposed homogenizing heat treatment provided a significant reduction in dendritic segregation. The difference in microhardness in dendrites and interdendritic spaces also decreased (Table 3) compared with the structure after casting without heat treatment. Microhardness was determined on a PMT-3 microhardness meter at a load of 5 gs. After the proposed heat treatment, the thin branches of the dendrites completely dissolved, and the larger branches of the dendrites acquire a globular shape. After etching, white patches were identified that represent the axis of the dendritic structure, and dark patches are the interdendritic patches.

The proposed steel composition and heat treatment method made it possible to provide a more uniform structure and fine grain in the steel structure compared to the prototype steel, which is ensured by additional alloying of Ti, Nb, and Al steel and the selected ratio of elements.

The proposed steel can be used as a high-strength non-magnetic corrosion-resistant material for special shipbuilding and drilling equipment. The proposed steel has undergone extensive laboratory research and is recommended for industrial testing.

Table 1 The chemical composition of the proposed and known steel Structure The content of elements, wt.% FROM Si Mn Cr Ni Mo V Nb Ti Ca Xie N Al S P Fe one 0,03 0.10 14.0 19.0 8.25 0.80 0.08 0.02 0.004 0.005 0.005 0.57 0.005 0.006 0.015 rest 2 0.04 0.25 15.0 20,0 8.45 0.98 0.12 0.09 0.01 0.01 0.008 0.60 0.008 0,055 0.015 rest 3 0.06 0.40 16,0 20.5 9.0 1.25 0.15 0.12 0,03 0.02 0,025 0.65 0.02 0.008 0.009 rest four 0.04 0.60 14.3 23.0 7.0 1.95 0.30 - - 0.01 0,025 0.48 - 0.005 0.010 rest

table 2 Mechanical properties of the proposed and known steels Steel composition σ _0.2 , MPa σ _V , MPa δ,% ψ,% KCV, J / cm ² Phase composition (magnetism) The average grain size, microns one 520 768 56 60 280 At 48 2 510 775 53 58 275 At 45 3 530 780 52 56 250 At 43 four 394 690 38 fifty 200 ϒ + á 54

Table 3 The microhardness of the proposed steel Heat treatment mode Imprint area Microhardness, N _µ No heat treatment (cast condition) Dark 356 Bright 318 Homogenizing heat treatment Dark 318 Bright 318

Literature

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Claims (3)

1. High-strength non-magnetic corrosion-resistant cast steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, nitrogen, cerium, calcium and iron, characterized in that it additionally contains niobium, titanium and aluminum in the following ratio of components, wt.%:
carbon 0.03-0.06 silicon 0.10-0.40 manganese 14.0-16.0 chromium 19.00-20.5 nickel 8.25-9.0 molybdenum 0.8-1.25 vanadium 0.08-0.15 niobium 0.02-0.12 nitrogen 0.57-0.65 titanium 0.004-0.03 cerium 0.005-0.02 calcium 0.005-0.02 aluminum 0.005-0.02 iron and impurities rest 2. Steel according to claim 1, characterized in that the ratio of the total content of vanadium, niobium, titanium and aluminum to the total content of carbon and nitrogen is 0.15-0.50. 3. The method of heat treatment of high-strength non-magnetic corrosion-resistant cast steel, including homogenizing treatment with heating and cooling, characterized in that the steel is treated according to claim 1, while the stepwise heating is carried out to a temperature of 850 ° C, and then to 950 ° C and then to 1100-1150 ° C, and cooling is carried out in water.