RU2415963C2 - Heat resistant steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel of ferrite class is used as heat resistant and corrosion resistant sheet material for fabrication of boiler, furnace, petrochemical and other high temperature equipment operating at temperature up to 1200°C. Steel contains carbon, silicon, manganese, chromium, nickel, aluminium, niobium, titanium, copper, calcium, cerium, sulphur, phosphorus, nitrogen and iron at the following ratio of components, wt %: carbon 0.006 - 0.03, silicon 0.10 - 0.30, manganese 0.15 - 0.50, chromium 16.50 - 18.50, nickel 0.05 - 0.50, aluminium 2.50 - 3.00, niobium 0.20 - 0.40, titanium 0.10 - 0.40, copper ≤ 0.15, calcium from over 0.01 to 0.025, cerium 0.005 - 0.03, sulphur 0.006 - 0.015, phosphorus 0.006 - 0.020, nitrogen 0.008 - 0.02, iron - the rest.

EFFECT: raised operational reliability of equipment and resource of operation at maintaining high processability at steel forming operation, high scale resistance.

2 cl, 4 tbl

Description

The invention relates to the field of metallurgy, in particular to iron-chromium-aluminum steels, in particular to steel of ferritic class, and can be used as heat-resistant and corrosion-resistant sheet material with a thickness of 3.0 to 22.0 mm

The invention can be most effectively used in the manufacture of boiler and furnace equipment, petrochemical equipment and other high-temperature equipment operating in aggressive gas environments at temperatures up to 1100 ° C, and in the air up to 1200 ° C. The steels of this purpose are subject to increased requirements of scale resistance, ductility, welding properties necessary to ensure the manufacturability of the final product.

Famous steel ferritic class 08X17T, 12X17, 08X18T1 and others in accordance with GOST 5623-72, used for these purposes. However, well-known steel grades do not provide the necessary level of basic physical, mechanical and technological properties and the necessary scale resistance at operating temperatures.

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

Carbon up to 0.03 Chromium 12.0-25.0 Nickel 5.0-18.00 Titanium 0.25-0.50 Aluminum 3.00-9.20 Molybdenum 0.8-6.00 Lanthanum + Yttrium up to 0.05 Iron rest

(see RF Patent RU 2352680 C1, class C22C 38/50)

The disadvantage of this steel is the instability of structures and the scatter of data on mechanical properties due to the large interval in the contents of chromium, nickel, molybdenum and aluminum. In addition, there is no data on the regulation of the content of impurities S, P, N, etc., since their presence leads to a decrease in scale resistance and manufacturability in the process of manufacturing products. When alloyed with chromium, titanium, aluminum and molybdenum at the upper level, steel is difficult to deform during hot rolling with the formation of defects and ductility and toughness at room temperature are sharply reduced.

The closest to the proposed steel in technical essence and the achieved result is heat-resistant steel (see RF Patent RU 2033466 C1, CL C22C 38/54) of the following composition (mass%):

Carbon 0.01-0.065 Silicon 0.10-0.80 Manganese 0.80-1.50 Chromium 8.50-16.00 Aluminum 0.05-0.25 Nickel 0.50-2.50 Titanium 0.05-0.25 Calcium 0.005-0.10 REM 0.005-0.10 Boron 0.005-0.05 Copper ≤ 0.50 Sulfur ≤0.03 Phosphorus 0.05-0.18 Nitrogen ≤0.03 Iron rest

The disadvantage of this steel is the large variation in the chromium content, leading to the production of various steel structures (martensitic, martensitic-ferritic or ferritic), which leads to unstable properties. In addition, the low content of chromium and aluminum reduces the heat resistance of steel. The increased content of carbon and nitrogen in the presence of boron, which is not uniformly distributed in steels (it is larger along grain boundaries than in the grain body), leads to a significant decrease in ductility and toughness.

The proposed heat-resistant ferritic steel contains carbon, silicon, manganese, chromium, aluminum, nickel, titanium, cerium, calcium, copper, sulfur, phosphorus, nitrogen and iron, characterized in that it additionally contains niobium in the following ratio of components (wt.%):

Carbon 0.006-0.03 Silicon 0.10-0.30 Manganese 0.15-0.50. Chromium 16.50-18.50 Nickel 0.05-0.50 Aluminum 2,50-3.00 Niobium 0.20-0.40 Titanium 0.10-0.40 Copper ≤0.15 Calcium from more than 0.01 to 0.025 Cerium 0.005-0.03 Sulfur 0.006-0.015 Phosphorus 0.006-0.020 Nitrogen 0.008-0.02 Iron rest

Moreover, the total amount of titanium and niobium should not be less than 0.35 wt.%. The proposed steel differs from the known one in that it additionally contains niobium (0.20-0.40 mass%), which, when combined with titanium, contributes to an increase in the heat-resistant properties of the proposed steel. The need for the joint introduction of niobium and titanium in an amount of more than 0.35 wt.% Is due to the nature of their effect on the properties of steel. Niobium in the presence of titanium and nitrogen forms complex finely dispersed nitrides and carbides that are evenly distributed over the grain body and along the boundaries, which positively affects grain refinement and increased ductility. The introduction of these elements in the indicated ratio with other elements improves the structural stability of steel and provides the necessary level of strength and plastic properties of rolled products, which increases the yield and increases the working capacity of the material in the products. In addition, titanium and niobium bind carbon and nitrogen to carbides, nitrides and carbonitrides, prevent the precipitation of chromium carbides and carbonitrides along the grain boundaries, which does not lead to depletion of the metal base with chromium and increases the heat resistance of steel. The introduction of niobium increases the total time to fracture due to a decrease in the propagation speed of cracks due to the uniform distribution of carbides along the grain boundaries, rather than the carbide network.

The combined introduction of these alloying elements in an amount of more than 0.35 wt.% Contributes to grain refinement, which leads to an increase in the technological plasticity of steel and a decrease in the cold brittleness threshold.

When the total content of niobium and titanium is lower than 0.35 wt.%, Their effect on the heat-resistant properties of steel is less effective, oxidation resistance decreases and strength properties and ductility are reduced.

The proposed steel differs from the known lower carbon content, 0.006-0.03, against 0.01-0.065 wt.% In the known steel, which provides high corrosion resistance and scale resistance.

When the carbon content is above the upper limit, the corrosion resistance and heat resistance decrease due to the increased precipitation of carbides and carbonitrides at the grain boundaries, as well as the ductility decreases sharply during hot and cold processing.

When the carbon content is below the specified alloying limits, the cost of smelting steel with an ultra-low carbon content increases sharply.

Manganese and silicon within specified limits ensures high-quality processes of smelting of the proposed steel. Manganese is a refining additive, it binds sulfur to stable dispersed sulfides and contributes to their more uniform distribution in the entire volume of steel. Silicon helps increase the scale resistance of steel (together with chromium).

The proposed steel differs from the known high chromium content, 16.5-18.5 wt.%, Against 8.5-16.0 wt.%, Which provides high corrosion resistance and scale resistance (together with aluminum).

When the chromium content is below the lower limit, the structure of the steel changes and the corrosion resistance and scale resistance decrease, and with an increase in the chromium content above the upper limit, there is no significant increase in scale resistance, but the processability decreases during hot and cold processing by reducing ductility.

Permissible impurities of nickel (up to 0.50 mass%) and copper (up to 0.15 mass%) essentially do not change the structural state and mechanical properties of steel, but slightly improve the state of the grain boundaries of ferrite, thereby increasing its ductility.

The proposed steel differs from the known high aluminum content, 2.50-3.00 wt.%, Against 0.05-0.25 wt.%, Which contributes to a significant increase in scale resistance of steel.

When the aluminum content is below the lower limit, the necessary scale resistance is not provided, and with an increase in the aluminum content above the upper limit, the ductility decreases and the deformability of the steel is complicated both in the hot and cold state.

The content of calcium and cerium within the stated limits in the proposed steel favorably changes the shape of non-metallic inclusions, cleans and strengthens grain boundaries, leading to an increase in ductility and toughness. Cerium in the presence of calcium improves oxidation resistance. With the total introduction of cerium and calcium in the indicated limits, the scale resistance of the proposed steel increases.

When the content of calcium and cerium is below the lower limit, their effect on ductility and toughness is not very effective, and when they are above the upper limit, the resistance to internal oxidation decreases and the ductility and toughness of the proposed steel are reduced.

Limiting the content of impurities of phosphorus and sulfur in the proposed steel helps to obtain higher values of ductility and scale resistance by reducing the number of fusible compounds of these impurities at the grain boundaries. With an increase in the content of fusible impurities of sulfur and phosphorus, in turn, scale resistance decreases due to an increase in the heterogeneity of the steel structure and ductility is significantly reduced.

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

Smelting was carried out in a 150-kg induction furnace with metal casting into ingots, which were rolled onto a 6 mm thick sheet to determine mechanical properties and heat resistance.

The proposed alloy underwent heat treatment in the following mode: heating to a temperature of 900 ° C, cooling to water.

Table 2 shows the mechanical properties of steel obtained after heat treatment in the above mode.

The results of the study of heat resistance are given in table 3, 4.

As can be seen from tables 2, 3 and 4, the proposed steel has a higher heat resistance compared to the prototype, higher mechanical properties, stability of the ferritic structure, a high level of hot and cold technological ductility. Using the proposed steel as a material for high-temperature equipment allows to increase operational reliability and provide an increased service life (boiler and furnace equipment) of oil refineries and chemical equipment at operating temperatures of 1100-1200 ° С. Improving the properties is provided by the selected composition of the steel.

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

Information sources

RF patent RU 2352680 C1, 09.24.2007.

RF patent RU 2033466 C1. 12/04/1991.

RF patent RU 2033460 C1, 12/14/1991.

RF patent RU 2040578 C1, 02/08/1993.

RF patent RU 2078844 C1, 12.19.1994.

RF patent RU 2222633 C1, 04.29.2004 GOST 5623-72.

Table 1 The chemical composition of the alloys No. p. FROM Si Mn Cr Ni Nb Ti Al Cu Sa Xie S R N Fe one 0.006 0.10 0.10 16.50 0.05 0.20 0.15 2,50 0.10 0.005 0.005 0.006 0.006 0.008 Ost. 2 0.02 0.20 0.35 17.6 0.30 0.25 0.30 2.65 0.15 0.012 0.02 0.010 0.005 0.01 Ost. 3 0,03 0.30 0.50 18.50 0.50 0.40 0.40 3.00 0.15 0,025 0,03 0.015 0.02 0.02 Ost. four 0.06 0.80 0.90 16,0 2.00 - 0.25 0.08 0.40 0.10 0,03 0,03 0.05 0,03 Ost.

table 2 The mechanical properties of steels Steel composition σ _0.2 , H / mm ² σ _b . N / mm ² δ,% KCU, J / cm ² one 355.0 495.0 35.0 210 2 358.0 515.0 32,0 215 3 365,0 520.0 35.5 200 four 320,0 430.0 24.0 95

Table 3 Heat resistance of steels in air Steel composition Temperature ° C Depth of corrosion, mm 1000 h 2000 h 3000 h 4000 h one 1200 0.007 0.012 0.015 0.018 2 0.007 0.013 0.015 0.019 3 0.006 0.011 0.014 0.019 four 0.012 0.019 0,028 0,036

Table 4 Heat resistance of steels in a sulfur-containing gas environment Steel composition Temperature ° C Depth of corrosion, mm 1000 h 2000 h 3000 h 4000 h one 1100 0.003 0.008 0.009 0.012 2 0.003 0.007 0.008 0.013 3 0.003 0.007 0.008 0.012 four 0.010 0.015 0,020 0,026

Claims (2)

1. Heat-resistant ferritic steel containing carbon, silicon, manganese, chromium, nickel, aluminum, niobium, titanium, copper, calcium, cerium, sulfur, phosphorus, nitrogen and iron, characterized in that it contains components in the following ratio, wt. %:
carbon 0.006-0.03 silicon 0.10-0.30 manganese 0.15-0.50 chromium 16.50-18.50 nickel 0.05-0.50 aluminum 2,50-3,00 niobium 0.20-0.40 titanium 0.10-0.40 copper ≤0.15 calcium from more than 0.01 to 0.025 cerium 0.005-0.03 sulfur 0.006-0.015 phosphorus 0.006-0.020 nitrogen 0.008-0.02 iron rest 2. Steel according to claim 1, characterized in that the total content of titanium and niobium is at least 0.35 wt.%.