RU2524465C1 - Refractory martensitic steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to refractory martensitic chromic steels used for making the large-diameter forged pieces of high hardness, durability and refractory properties at 650°C and for production of steam pipes and boilers operated at 650°C. Steel contains the following elements, in wt %: carbon 0.015-0.05, silicon 0.10-0.20, manganese 0.45-0.70, chromium 9.10-12.00, nickel ≤ 0.30, tungsten 1.00-1.70, molybdenum 0.65-0.90, vanadium 0.15-0.30, niobium 0.15-0.30, nitrogen 0.025-0.25, boron 0.001-0.003, sulphur ≤ 0.006, phosphorus ≤ 0.008, aluminium 0.001-0.003, copper ≤ 0,30, cobalt 4.00-5.00, zirconium nitride 0.05-0.50, calcium 0.005-0.02, cerium 0.005-0.03, iron making the rest. Zirconium nitride in steel represents nanosized particles.

EFFECT: higher hardness and durability and refractory properties.

2 cl, 3 tbl

Description

The invention relates to the field of metallurgy, in particular to heat-resistant chrome steels of martensitic class containing 8-13% Cr, which can be used for the manufacture of forgings of large diameter rotors with high strength, endurance and heat-resistant properties at a temperature of 650 ° C, and also manufacturing steam pipelines and boilers of power plants with operating temperatures up to 650 ° C.

The closest in technical essence and the achieved result is martensitic class heat-resistant steel for the manufacture of elements of power plants, in particular boilers and steam lines, containing carbon, silicon, manganese, chromium, nickel, tungsten, molybdenum, cobalt, vanadium, niobium, nitrogen, boron, sulfur, phosphorus, aluminum, copper and iron in the following ratio of components, wt.%: carbon no more than 0.02, silicon 0.15-0.2, manganese 0.45-0.6, chromium 8.5-9, 0, nickel not more than 0.05, tungsten 1.7 -1.75, molybdenum 0.5-0.6, cobalt 2.8-3.2, vanadium 0.18-0.23, niobium 0.05- 0.08 az from 0.05-0.07, boron 0.006-0.008, sulfur no more than 0.01, phosphorus no more than 0.01, copper 0.01-0.05, aluminum no more than 0.003, iron - the rest.

(RU 2437956, C22C 38/54, C22C 38/32)

However, the known steel does not have sufficiently high mechanical properties and heat resistance, which limits its use in power plants operating on super supercritical steam parameters.

The objective and technical result of the invention is to increase the strength, endurance and heat resistance of steel.

The technical result is achieved in that the heat-resistant steel of the martensitic class contains carbon, silicon, manganese, chromium, nickel, tungsten, molybdenum, vanadium, niobium, nitrogen, boron, sulfur, phosphorus, aluminum, copper, cobalt, zirconium nitride, calcium, cerium and iron, in the following ratio of components, wt.%:

Carbon 0.015-0.05 Silicon 0.10-0.20 Manganese 0.45-0.70 Chromium 9.10-12.00 Nickel ≤0.30 Tungsten 1.00 - 1.70 Molybdenum 0.65-0.90 Vanadium 0.15-0.30 Niobium 0.15-0.30 Nitrogen 0.025-0.25 Boron 0.001-0.003 Sulfur ≤0.006 Phosphorus ≤0.008 Aluminum 0.001-0.003 Copper ≤0.30 Cobalt 4.00-5.00 Zirconium nitride 0.05-0.50 Calcium 0.005-0.02 Cerium 0.005-0.03 Iron rest

The technical result is also achieved in that the steel contains zirconium nitride in the form of particles with nanoscale dispersion.

The proposed steel differs from the known rational carbon content of 0.015-0.05 wt.%, Which is optimal to ensure high processability and contributes to high strength and heat resistance.

When the carbon content is below 0.015 wt.%, Its effect on the technological and service properties has become ineffective, but smelting processes are complicated, and when the carbon content is above 0.05 wt.%, The coalescence of carbides and depletion of the solid solution of Mo, Cr and V are accelerated, which reduces the strength properties and heat resistance of steel.

The optimum chromium content of 9.10-12.00 wt.% Provides high hardenability and higher heat resistance.

When the chromium content is lower than 9.1 wt.%, Its effect on hardenability is less effective, and when the chromium content is higher than 12.0 wt.%, Hardenability and heat resistance slightly increase, but at the same time there is the possibility of the formation of δ-ferrite. To achieve maximum strength, steel should be completely martensitic after cooling in air, since any content of δ-ferrite reduces its strength: with an increase in the amount of δ-ferrite, embrittlement of steel increases under prolonged exposure to elevated temperature.

The molybdenum content of 0.65-0.90 wt.% Provides an increase in hardenability, strength and heat resistance of steel, since molybdenum is in solid solution, which leads to additional hardening without reducing ductility and prevents the development of temper brittleness. The proposed range of molybdenum content helps to suppress the release of Laves phases, which coagulate rapidly at high temperatures, which leads to a decrease in the characteristics of heat resistance.

The additional presence of calcium and cerium in the steel composition, combined with a balanced residual aluminum content, favorably changes the shape of non-metallic inclusions, cleans and strengthens grain boundaries, increases ductility, toughness and heat resistance, which leads to an increase in the service and technological properties of steel.

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

The introduction of finely dispersed zirconium nitrides with nanoscale dispersion into the composition of the steel ensures the formation of a large number of crystallization centers uniformly distributed in the metal volume.

During solidification, chemically stable particles of zirconium nitride, being in the melt, have increased resistance to dissociation and will be centers of crystallization of austenitic grains, which will significantly grind the primary austenitic grain, increase the border area of austenitic grains and significantly reduce the amount and dispersion of vanadium carbides and nitrides and niobium falling along the boundaries of austenitic grains. All this leads to an increase in the strength characteristics of steel, as well as indicators of ductility and toughness. Zirconium nitride also plays the role of additional phase nuclei released during creep, due to which a finer dispersed phase distribution is formed, which increases the heat resistance of steel.

Microalloying steel with boron and nitrogen increases the resistance of steel to deformation during creep due to the formation of boron nitrides. Boron segregates along grain boundaries, mainly former austenitic, which, suppressing grain-boundary slippage, increases the time to failure. In addition, boron increases stress corrosion resistance and eliminates the adverse effect of high vanadium content on scale resistance. Boron forms boron nitride nanoparticles in the body of grains and along dislocation walls, which makes it possible to raise the operating temperature due to the effect of stabilization of the dislocation structure. Boron nanoparticles also increase the effect of zirconium nitride nanoparticles on the heat resistance of steel.

The inventive steel implements the mechanism of nanoscale self-regulation of the structure under long-term operation, which consists in fixing dislocations with nanoscale precipitates (no more than 20-30 nm in size) of boron nitride and zirconium nitride, which are also highly stable when exposed to elevated temperatures and high voltages, which significantly increases the heat resistance become.

The limitation of the content of sulfur and phosphorus impurities to 0.006 and 0.008 wt.%, Respectively, 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 its heat resistance.

The increased nitrogen content of 0.025-0.25 wt.% Helps to increase the strength of steel due to the formation of nitrides and carbonitrides of vanadium, niobium and chromium. Highly dispersed nitrides and carbonitrides of these elements inhibit grain growth upon heating, which helps to maintain high impact strength. Such a nitrogen content ensures the absence of δ ferrite in the steel structure, the presence of which reduces heat resistance.

The increased cobalt content of 4.00-5.00 wt.% Helps to suppress the formation of δ-ferrite during austenitization of steels with a chromium content of 8-12% wt.% And significantly affects the release of dispersed hardening particles during tempering. The total amount of precipitates such as carbonitrides and carbides increases with increasing cobalt content. The change in the density of the precipitates is especially pronounced in the range of cobalt content within the stated limits.

The increased vanadium content of 0.15-0.30 wt.% Contributes to the grinding of grain, reduces the tendency of steel to overheat and increases the resistance of martensite to tempering.

The proposed steel can increase the copper content of not more than 0.30 wt.%, Which makes it possible to use a cheaper initial charge (since copper is present in the scrap metal).

Comparative tests of known steel and steel according to the invention are presented in tables 1-3.

The smelting was carried out in an induction furnace, with the casting of metal on ingots, from which samples were made after forging to determine the mechanical properties and heat resistance.

Table 2 shows the mechanical properties of steel samples obtained after the following heat treatment: quenching from 1100 ° C in oil, tempering at a temperature of 750 ° 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 at room temperature and according to GOST 9651-84 at elevated temperatures. As a criterion for heat resistance, tests for long-term strength were used, which were carried out according to GOST 10145-81 (Table 3).

As can be seen from the data presented, the steel according to the invention has higher mechanical properties and heat resistance than the known steel. The proposed steel after heat treatment has a martensitic structure without the presence of δ-ferrite, which positively affects the heat resistance of steel.

The service characteristics of the steel according to the invention allow it to be used as a structural material for parts of thermal turbines with a working temperature of up to 650 ° C.

Table 1 The chemical composition of steels The concentration of components, wt.% Steel according to the invention Famous steel one 2 3 four FROM 0.015 0,03 0.05 0.02 Si 0.10 0.15 0.20 0.10 Mn 0.40 0.60 0.70 0.45 S 0.002 0.004 0.006 0.01 R 0.003 0.002 0.007 0.01 Cr 9.10 10.50 12.00 9.00 Ni 0.30 0.20 0.10 0.05 Mo 0.65 0.70 0.90 0.60 W 1.00 1.20 1.70 1.70 With 4.00 4,50 5.00 3.00 Cu 0.10 0.25 0.30 0.05 V 0.15 0.25 0.30 0.20 Nb 0.15 0.30 0.20 0.05 N 0,025 0.10 0.25 0,07 Al 0.001 0.002 0.003 0.003 AT 0.001 0.002 0.003 0.007 Zrn 0.05 0.40 0.50 - Xie 0.005 0.010 0,030 - Sa 0.005 0.010 0,020 - Fe rest rest rest rest

table 2 The mechanical properties of the known steel and steel according to the invention Steel composition T _isp., ° C σ _0.2 , N / mm ² σ _b , N / mm ² δ,% twenty 850 1050 fifteen one 650 500 550 twenty 700 460 500 25 twenty 950 1100 fourteen 2 650 510 560 twenty 700 465 510 24 twenty 1000 1150 fifteen 3 650 520 570 twenty 700 470 575 25 twenty 700 790 fourteen four 650 320 440 fifteen 700 130 260 25 Table 3 Long-term strengths of steels depending on test temperature Steel composition T _isp , ° C Long-term strength, N / mm ² , during 10 ⁵ hours one 650 120 2 650 123 3 650 125 four 650 108

Claims (2)

1. Heat-resistant steel of the martensitic class containing carbon, silicon, manganese, chromium, nickel, tungsten, molybdenum, vanadium, niobium, nitrogen, boron, sulfur, phosphorus, aluminum, copper, cobalt and iron, characterized in that it additionally contains nitride zirconium, cerium and calcium in the following ratio of components, wt.%:
Carbon 0.015-0.05 Silicon 0.10-0.20 Manganese 0.45-0.70 Chromium 9.10-12.00 Nickel ≤0.30 Tungsten 1.00-1.70 Molybdenum 0.65-0.90 Vanadium 0.15-0.30 Niobium 0.15-0.30 Nitrogen 0.025-0.25 Boron 0.001-0.003 Sulfur ≤0.006 Phosphorus ≤0.008 Aluminum 0.001-0.003 Copper ≤0.30 Cobalt 4.00-5.00 Zirconium nitride 0.05-0.50 Calcium 0.005-0.02 Cerium 0.005-0.03 Iron rest 2. Steel according to claim 1, characterized in that it contains zirconium nitride in the form of particles with nanoscale dispersion.