RU2441092C1 - Heat-resistant steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to heat-resistant steels used for casting steam turbine parts, tubes and valves and accessories by electro slag melting and spun casting at 540 - 580°C. Steel contains carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, copper, aluminium, sulfur, nitrogen, phosphorus, calcium, cerium and iron at the following ratio of components, in wt %: carbon - 0.11.015, silicon - 0.17-0.37, manganese - 0.40-0.70, chromium, nickel - ≤ 0.30, molybdenum 0.80-1.10, vanadium 0.20-0.35, copper - ≤ 0.30, aluminium - 0.001-0.008, sulfur ≤ 0.006, nitrogen 0.005-0.012, phosphorus - ≤ 0.008, calcium - 0.005-0.02, cerium - 0.005-0.03, iron making the rest.

EFFECT: higher mechanical strength and scale resistance.

2 cl, 3 tbl

Description

The invention relates to the field of metallurgy, in particular to heat-resistant steels, in particular to the creation of steels that can be used for casting parts of turbines, parts of valves and other equipment in other industries.

The invention can be most effectively used in the manufacture of fittings by electroslag remelting (ESR), as well as centrifugally cast pipes and parts for steam turbines with a capacity of 300-1200 MW with operating conditions at temperatures of 540-580 ° C and pressure from 2.5 to 5.0 MPa. The invention can also be used for petrochemical equipment.

Known steel used for such purposes, consisting of the following components (wt.%):

Carbon 0.10-0.28 Silicon 0.05-0.37 Manganese 0.17-0.50 Chromium 2,50-3,30 Molybdenum 0.60-0.80 Vanadium 0.20-0.40 Nickel 0.05-0.40 Copper 0.03-0.30 Aluminum 0.01-0.10 Nitrogen 0.005-0.02 Calcium 0.001-0.005 Sulfur 0.002-0.015 Phosphorus 0.002-0.015 Tin 0.001-0.004 Arsenic 0.002-0.005 Antimony 0.001-0.005 Zirconium 0.003-0.010 Niobium 0.001-0.030 Sodium 0.001-0.005 Iron rest

(see RF Patent RU 2241061 C2, class C22C 38/60)

The disadvantage of this steel is the poor casting characteristics and instability of toughness due to the spread of the alloying interval, especially in terms of carbon content.

Casting steel of the following composition is also known (wt.%):

Carbon 0.11-0.13 Silicon 0.17-0.37 Manganese 0.90-1.40 Chromium 0.80-2.50 Nickel 0.20-0.60 Molybdenum 0.10-0.80 Vanadium 0.03-0.14 Rare earth oxides of metals 0.10-0.50 Niobium 0.01-0.06 Iron rest

(see RF Patent RU 2083716 C1, class C22C 38/48)

The disadvantage of this steel is the lack of regulation on impurities (S, P, etc.), which significantly reduces the quality of castings and the spread of data on mechanical properties due to the large alloying interval for chromium and molybdenum. This steel also has reduced heat resistance, especially when the ingredients are kept at a lower level.

Closest to the proposed alloy in technical essence and the achieved result is steel (see British Patent 1558731, class C22C 38/60,) of the following composition (wt.%):

Carbon 0.05-0.20 Silicon 0.01-0.50 Manganese 0.60-2.00 Chromium ≤ 0.80 Nickel 0.10-0.60 Molybdenum 0.10-0.80 Vanadium 0.01-0.15 Niobium 0.01-0.15 Zirconium 0.01-0.10 Titanium 0.01-0.10 Boron 0.0005-0.005 Copper 0.20-0.60 Aluminum 0.01-0.10 Sulfur ≤0.002 REM 0.008-0.03 Iron rest

The disadvantages of this steel are low yield strength and strength and low scale resistance when the ingredients are kept at a lower level. In addition, when the carbon content at the upper level of the prototype steel (0.20 wt.%), The steel is prone to the formation of hot cracks in the process of production of pipe blanks by ESR.

EFFECT: obtaining heat-resistant steel with high mechanical properties and scale resistance, which ensures an increase in the operating temperature of turbines to 580 ° C. This result is achieved by the fact that the proposed steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, copper, aluminum, sulfur, cerium and iron, according to the proposal additionally contains phosphorus, nitrogen and calcium in the following ratio of components, (wt. .%):

Carbon 0.11-0.15 Silicon 0.17-0.37 Manganese 0.40-0.70 Chromium 1.10-2.00 Nickel ≤0.30 Molybdenum 0.80-1.10 Vanadium 0.20-0.35 Aluminum 0.001-0.008 Copper ≤0.30 Sulfur ≤0.006 Phosphorus ≤0.008 Nitrogen 0.005-0.012 Calcium 0.005-0.02 Cerium 0.005-0.03 Iron rest

The total content of residual aluminum, calcium and cerium is 0.010-0.05 wt.%.

The proposed steel is characterized by an optimal carbon content of 0.11-0.15 wt.%, Against 0.05-0.20 wt.% In the prototype, which provides high adaptability for ESR, centrifugal casting and welding. However, this carbon content in the proposed steel provides the necessary strength and scale resistance.

When the carbon content is below the lower limit, its effect on the service properties has become ineffective, since when the carbon content is 0.05 wt.% (As in the prototype), the mechanical properties are reduced and the technology is complicated by centrifugal casting, and when the carbon content is at the upper level of the steel of the prototype ( 0.20 wt.%) There is an increase in strength characteristics with a simultaneous sharp decrease in toughness due to an increase in the number of carbides and their coalescence and depletion of a solid solution of Mo, Cr and V, which reduces the oxidation resistance of a hundred whether. In addition, this carbon content leads to the formation of hot cracks in the process of production of pipe blanks by ESR.

The proposed steel is characterized by an optimum silicon content of 0.17-0.37 wt.%, Against 0.01-0.50 wt.% In the known steel, which is quite enough for the deoxidation of steel.

When the silicon content is below the lower limit, its effect on the properties of steel is ineffective, and when the silicon content is above the upper limit, the strength and scale resistance increase, but the toughness and processability in the ESR process are reduced.

The proposed steel differs from the known high chromium content of 1.10-2.00 wt.%, Against not more than 0.80 wt.%, Which provides high hardenability and higher scale resistance.

When the chromium content is below the lower limit, its effect on hardenability is less effective, and when the chromium content is above the upper limit, hardenability and scale resistance increases slightly, but the cost also increases.

The proposed steel differs from the known high content of molybdenum (0.80-1.10 wt.%), Against 0.10-0.80 wt.%, Which helps to increase hardenability, strength and scale resistance, without reducing ductility and preventing the development of temper brittleness .

When the molybdenum content is below the lower limit, its effect on strength, ductility and scale resistance is less effective, and when the molybdenum content is above the upper limit, some increase in strength, ductility and scale resistance contradicts economic feasibility.

The proposed steel differs from the known one in that the total content of aluminum, calcium and cerium is 0.010-0.05 wt.%.

The introduction of a limited content of active elements of calcium and cerium into the composition of the steel, combined with a balanced content of residual aluminum, favorably changes the shape of non-metallic inclusions, cleans and strengthens grain boundaries, increases ductility, toughness and scale resistance, which leads to an increase in the service and technological properties of steel. The lower level of aluminum content is determined by the requirement to ensure deoxidizability of steel, and the upper level is determined by the requirement to ensure a given level of ductility of steel.

Cerium in the presence of calcium improves oxidation resistance. With the total introduction of cerium and calcium within the specified limits, the scale resistance of the proposed steel increases.

When the content of calcium and cerium is lower than the lower limit of the total content, their effect on heat resistance is ineffective, and if they are higher than the upper limit of the total content, the oxidation resistance decreases and the heat resistance of the proposed steel decreases.

The residual aluminum content in the steel is 0.001-0.008 wt.%. When the residual aluminum content is below the lower limit, effective deoxidation of steel is not ensured, which leads to an increase in the number of oxide inclusions and a decrease in the strength properties of steel. With an increase in the residual aluminum content above the upper limit, the characteristics of heat resistance and toughness of steel decrease, which is due to the additional release of aluminum nitrides at the grain boundary.

The proposed steel differs from the known one by limiting the content of phosphorus impurities to 0.008 wt.%, Against no restrictions in the prototype steel, which helps to obtain higher values of ductility and toughness; With an increase in the content of fusible sulfur and phosphorus impurities above the stated limits, the heterogeneity of the steel structure sharply increases, which in turn reduces the heat resistance of steel. With an increase in the carbon content in steel above the stated value, more intense formation of phosphorus segregation occurs, which combine and form a network along the boundaries of primary austenitic grains, which leads to a weakening of intercrystalline bonds. Manganese is also able to enhance the embrittlement effect of phosphorus, therefore, in the production of steels containing manganese, it is necessary to strive to ensure minimum concentrations of phosphorus in steel. The presence in the composition of the proposed steel nickel and molybdenum significantly reduces the harmful effect of phosphorus on the properties of steel.

The proposed steel differs from the known additional nitrogen content of 0.005-0.012 wt.%, Which contributes to an increase in strength due to the formation of nitrides and carbonitrides of vanadium and chromium. Highly dispersed nitrides and carbonitrides of these elements inhibit grain growth upon heating, which helps to maintain high impact strength.

When the nitrogen content is below the lower limit, its effect on the strength and toughness of this steel is ineffective, and when the nitrogen content is above the upper limit, the strength slightly increases, but the toughness decreases, due to the enrichment of grain boundaries with carbides and carbonitrides.

The proposed steel differs from the known high content of vanadium 0.20-0.35 wt.%, Against 0.01-0.15 wt.%. Vanadium contributes to the grinding of grain, reduces the tendency of steel to overheat and increases the resistance of martensite to tempering. Vanadium enhances strength and heat resistance. The upper limit of the content of vanadium is 0.35 wt.%, Due to the need for the required level of ductility of steel, and the lower - respectively 0.20 wt.% - providing the required level of strength of this steel.

When the content of vanadium is below the lower limit, its effect on the strength and toughness of this steel is ineffective, and when the content of vanadium is above the upper limit, the strength and toughness decrease, which is associated with the enrichment of grain boundaries with vanadium carbides and carbonitrides.

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

The smelting was carried out in a 150-kg induction furnace, with metal casting on cast electrodes, followed by re-melting the ESR into pipe blanks with a diameter of 180 mm, a length of 350 mm in a laboratory setup, from which samples were made to determine mechanical properties and scale resistance.

Table 2 shows the mechanical properties obtained after heat treatment: homogenization of 1000 ° C, cooling in air, quenching from 980 ° C in oil, tempering at a temperature of 650 ° C, cooling in air.

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 Menage samples according to GOST 9454-78.

Tests for heat resistance (scale resistance) were carried out according to generally accepted methods (table 3).

As can be seen from tables 2 and 3, the proposed steel has higher mechanical properties and scale resistance than steel prototype. In addition, the proposed steel after heat treatment has a martensitic (martensite tempering) structure, which positively affects the heat resistance of steel.

Using the proposed steel as a material for casting ESR of thermal turbines can increase the operating temperature of the turbines to 580 ° C.

The proposed steel has undergone extensive laboratory research at OAO NPI TsNIITMASH and is recommended for industrial testing.

Literature:

1. RU 94018908 A1, C22C 38/00, 05/05/1994.

2. RU 93025219 A, C22C 38/28, 04/28/1993.

3. RU 2081199 C1, C22C 38/26, 07/19/1995.

4. RU 2336330 C1, C21D 8/10, C22C 38/60, C22C 38/48, 12.25.2006.

5. RU 2338796 C2, C21D 8/10, C22C 38/60, C22C 38/48, 12/18/2006.

6. RU 2083716 C1, C22C 38/48, 02/10/1993.

7. RU 2241061 C2, cl. C22C 38/60, 09/07/2001.

8. UK patent 1558731, CL C7A.

9. Japanese Patent JP 2010209471 (A).

10. EP 2192203 (A1).

table 2 Mechanical properties of the proposed and known steels Steel composition T _use. ° C σ _0.2 , σ _b δ, φ KCU N / mm ² % J / cm ² one twenty 550 750 22.0 55.0 130.0 2 twenty 560 770 21.0 52.0 1250 3 twenty 555 765 24.0 53.0 125.0 four twenty 520 655 18.0 60.0 95.0

Table 3 Heat resistant offered and famous steels Steel composition Wednesday T _isp., ° C Corrosion rate, mm / year Test base, h one Steam 610 0.09 3000 2 0.05 3 0.06 four 0.28 one Air 600 0.16 3000 2 0.15 3 0.16 four 0.41

Claims (2)

1. Heat-resistant steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, copper, aluminum, sulfur, cerium and iron, characterized in that it additionally contains phosphorus, nitrogen and calcium in the following ratio, wt.%:
carbon 0.11-0.15 silicon 0.17-0.37 manganese 0.40-0.70 chromium 1.10-2.00 nickel ≤0.30 molybdenum 0.80-1.10 vanadium 0.20-0.35 aluminum 0.001-0.008 copper ≤0.30 sulfur ≤0.006 phosphorus ≤0.008 nitrogen 0.005-0.012 calcium 0.005-0.02 cerium 0.005-0.03 iron rest 2. Steel according to claim 1, characterized in that the total content of residual aluminum, calcium and cerium is 0.010-0.05 wt.%.